// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
// #define INTERACTIVE

#include <bits/stdc++.h>
using namespace std;

namespace templates {
// type
using ll  = long long;
using ull = unsigned long long;
using Pii = pair<int, int>;
using Pil = pair<int, ll>;
using Pli = pair<ll, int>;
using Pll = pair<ll, ll>;
template <class T>
using pq = priority_queue<T>;
template <class T>
using qp = priority_queue<T, vector<T>, greater<T>>;
// clang-format off
#define vec(T, A, ...) vector<T> A(__VA_ARGS__);
#define vvec(T, A, h, ...) vector<vector<T>> A(h, vector<T>(__VA_ARGS__));
#define vvvec(T, A, h1, h2, ...) vector<vector<vector<T>>> A(h1, vector<vector<T>>(h2, vector<T>(__VA_ARGS__)));
// clang-format on

// for loop
#define fori1(a) for (ll _ = 0; _ < (a); _++)
#define fori2(i, a) for (ll i = 0; i < (a); i++)
#define fori3(i, a, b) for (ll i = (a); i < (b); i++)
#define fori4(i, a, b, c) for (ll i = (a); ((c) > 0 || i > (b)) && ((c) < 0 || i < (b)); i += (c))
#define overload4(a, b, c, d, e, ...) e
#define fori(...) overload4(__VA_ARGS__, fori4, fori3, fori2, fori1)(__VA_ARGS__)

// declare and input
// clang-format off
#define INT(...) int __VA_ARGS__; inp(__VA_ARGS__);
#define LL(...) ll __VA_ARGS__; inp(__VA_ARGS__);
#define STRING(...) string __VA_ARGS__; inp(__VA_ARGS__);
#define CHAR(...) char __VA_ARGS__; inp(__VA_ARGS__);
#define DOUBLE(...) double __VA_ARGS__; STRING(str___); __VA_ARGS__ = stod(str___);
#define VEC(T, A, n) vector<T> A(n); inp(A);
#define VVEC(T, A, n, m) vector<vector<T>> A(n, vector<T>(m)); inp(A);
// clang-format on

// const value
const ll MOD1   = 1000000007;
const ll MOD9   = 998244353;
const double PI = acos(-1);

// other macro
#if !defined(RIN__LOCAL) && !defined(INTERACTIVE)
#define endl "\n"
#endif
#define spa ' '
#define len(A) ll(A.size())
#define all(A) begin(A), end(A)

// function
vector<char> stoc(string &S) {
    int n = S.size();
    vector<char> ret(n);
    for (int i = 0; i < n; i++) ret[i] = S[i];
    return ret;
}
string ctos(vector<char> &S) {
    int n      = S.size();
    string ret = "";
    for (int i = 0; i < n; i++) ret += S[i];
    return ret;
}

template <class T>
auto min(const T &a) {
    return *min_element(all(a));
}
template <class T>
auto max(const T &a) {
    return *max_element(all(a));
}
template <class T, class S>
auto clamp(T &a, const S &l, const S &r) {
    return (a > r ? r : a < l ? l : a);
}
template <class T, class S>
inline bool chmax(T &a, const S &b) {
    return (a < b ? a = b, 1 : 0);
}
template <class T, class S>
inline bool chmin(T &a, const S &b) {
    return (a > b ? a = b, 1 : 0);
}
template <class T, class S>
inline bool chclamp(T &a, const S &l, const S &r) {
    auto b = clamp(a, l, r);
    return (a != b ? a = b, 1 : 0);
}

template <typename T>
T sum(vector<T> &A) {
    T tot = 0;
    for (auto a : A) tot += a;
    return tot;
}

template <typename T>
vector<T> compression(vector<T> X) {
    sort(all(X));
    X.erase(unique(all(X)), X.end());
    return X;
}

// input and output
namespace io {
// __int128_t
std::ostream &operator<<(std::ostream &dest, __int128_t value) {
    std::ostream::sentry s(dest);
    if (s) {
        __uint128_t tmp = value < 0 ? -value : value;
        char buffer[128];
        char *d = std::end(buffer);
        do {
            --d;
            *d = "0123456789"[tmp % 10];
            tmp /= 10;
        } while (tmp != 0);
        if (value < 0) {
            --d;
            *d = '-';
        }
        int len = std::end(buffer) - d;
        if (dest.rdbuf()->sputn(d, len) != len) {
            dest.setstate(std::ios_base::badbit);
        }
    }
    return dest;
}

// vector<T>
template <typename T>
istream &operator>>(istream &is, vector<T> &A) {
    for (auto &a : A) is >> a;
    return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<T> &A) {
    for (size_t i = 0; i < A.size(); i++) {
        os << A[i];
        if (i != A.size() - 1) os << ' ';
    }
    return os;
}

// vector<vector<T>>
template <typename T>
istream &operator>>(istream &is, vector<vector<T>> &A) {
    for (auto &a : A) is >> a;
    return is;
}
template <typename T>
ostream &operator<<(ostream &os, vector<vector<T>> &A) {
    for (size_t i = 0; i < A.size(); i++) {
        os << A[i];
        if (i != A.size() - 1) os << endl;
    }
    return os;
}

// pair<S, T>
template <typename S, typename T>
istream &operator>>(istream &is, pair<S, T> &A) {
    is >> A.first >> A.second;
    return is;
}
template <typename S, typename T>
ostream &operator<<(ostream &os, pair<S, T> &A) {
    os << A.first << ' ' << A.second;
    return os;
}

// vector<pair<S, T>>
template <typename S, typename T>
istream &operator>>(istream &is, vector<pair<S, T>> &A) {
    for (size_t i = 0; i < A.size(); i++) {
        is >> A[i];
    }
    return is;
}
template <typename S, typename T>
ostream &operator<<(ostream &os, vector<pair<S, T>> &A) {
    for (size_t i = 0; i < A.size(); i++) {
        os << A[i];
        if (i != A.size() - 1) os << endl;
    }
    return os;
}

// tuple
template <typename T, size_t N>
struct TuplePrint {
    static ostream &print(ostream &os, const T &t) {
        TuplePrint<T, N - 1>::print(os, t);
        os << ' ' << get<N - 1>(t);
        return os;
    }
};
template <typename T>
struct TuplePrint<T, 1> {
    static ostream &print(ostream &os, const T &t) {
        os << get<0>(t);
        return os;
    }
};
template <typename... Args>
ostream &operator<<(ostream &os, const tuple<Args...> &t) {
    TuplePrint<decltype(t), sizeof...(Args)>::print(os, t);
    return os;
}

// io functions
void FLUSH() {
    cout << flush;
}

void print() {
    cout << endl;
}
template <class Head, class... Tail>
void print(Head &&head, Tail &&...tail) {
    cout << head;
    if (sizeof...(Tail)) cout << spa;
    print(std::forward<Tail>(tail)...);
}

template <typename T, typename S>
void prisep(vector<T> &A, S sep) {
    int n = A.size();
    for (int i = 0; i < n; i++) {
        cout << A[i];
        if (i != n - 1) cout << sep;
    }
    cout << endl;
}
template <typename T, typename S>
void priend(T A, S end) {
    cout << A << end;
}
template <typename T>
void prispa(T A) {
    priend(A, spa);
}
template <typename T, typename S>
bool printif(bool f, T A, S B) {
    if (f)
        print(A);
    else
        print(B);
    return f;
}

template <class... T>
void inp(T &...a) {
    (cin >> ... >> a);
}

} // namespace io
using namespace io;

// read graph
vector<vector<int>> read_edges(int n, int m, bool direct = false, int indexed = 1) {
    vector<vector<int>> edges(n, vector<int>());
    for (int i = 0; i < m; i++) {
        INT(u, v);
        u -= indexed;
        v -= indexed;
        edges[u].push_back(v);
        if (!direct) edges[v].push_back(u);
    }
    return edges;
}
vector<vector<int>> read_tree(int n, int indexed = 1) {
    return read_edges(n, n - 1, false, indexed);
}

template <typename T = long long>
vector<vector<pair<int, T>>> read_wedges(int n, int m, bool direct = false, int indexed = 1) {
    vector<vector<pair<int, T>>> edges(n, vector<pair<int, T>>());
    for (int i = 0; i < m; i++) {
        INT(u, v);
        T w;
        inp(w);
        u -= indexed;
        v -= indexed;
        edges[u].push_back({v, w});
        if (!direct) edges[v].push_back({u, w});
    }
    return edges;
}
template <typename T = long long>
vector<vector<pair<int, T>>> read_wtree(int n, int indexed = 1) {
    return read_wedges<T>(n, n - 1, false, indexed);
}

// yes / no
namespace yesno {

// yes
inline bool yes(bool f = true) {
    cout << (f ? "yes" : "no") << endl;
    return f;
}
inline bool Yes(bool f = true) {
    cout << (f ? "Yes" : "No") << endl;
    return f;
}
inline bool YES(bool f = true) {
    cout << (f ? "YES" : "NO") << endl;
    return f;
}

// no
inline bool no(bool f = true) {
    cout << (!f ? "yes" : "no") << endl;
    return f;
}
inline bool No(bool f = true) {
    cout << (!f ? "Yes" : "No") << endl;
    return f;
}
inline bool NO(bool f = true) {
    cout << (!f ? "YES" : "NO") << endl;
    return f;
}

// possible
inline bool possible(bool f = true) {
    cout << (f ? "possible" : "impossible") << endl;
    return f;
}
inline bool Possible(bool f = true) {
    cout << (f ? "Possible" : "Impossible") << endl;
    return f;
}
inline bool POSSIBLE(bool f = true) {
    cout << (f ? "POSSIBLE" : "IMPOSSIBLE") << endl;
    return f;
}

// impossible
inline bool impossible(bool f = true) {
    cout << (!f ? "possible" : "impossible") << endl;
    return f;
}
inline bool Impossible(bool f = true) {
    cout << (!f ? "Possible" : "Impossible") << endl;
    return f;
}
inline bool IMPOSSIBLE(bool f = true) {
    cout << (!f ? "POSSIBLE" : "IMPOSSIBLE") << endl;
    return f;
}

// Alice Bob
inline bool Alice(bool f = true) {
    cout << (f ? "Alice" : "Bob") << endl;
    return f;
}
inline bool Bob(bool f = true) {
    cout << (f ? "Bob" : "Alice") << endl;
    return f;
}

// Takahashi Aoki
inline bool Takahashi(bool f = true) {
    cout << (f ? "Takahashi" : "Aoki") << endl;
    return f;
}
inline bool Aoki(bool f = true) {
    cout << (f ? "Aoki" : "Takahashi") << endl;
    return f;
}

} // namespace yesno
using namespace yesno;

} // namespace templates
using namespace templates;

template <typename T>
struct BIT {
    int n;
    std::vector<T> tree;

    BIT(int n) : n(n) {
        tree.assign(n + 1, T(0));
    }
    BIT() = default;
    T _sum(int i) {
        i++;
        T res = T(0);
        while (i > 0) {
            res += tree[i];
            i -= i & -i;
        }
        return res;
    }

    T sum(int l, int r) {
        return _sum(r - 1) - _sum(l - 1);
    }

    T sum(int r) {
        return _sum(r - 1);
    }

    T get(int i) {
        return _sum(i) - _sum(i - 1);
    }

    void add(int i, T x) {
        i++;
        while (i <= n) {
            tree[i] += x;
            i += i & -i;
        }
    }

    int lower_bound(T x) {
        int pos  = 0;
        int plus = 1;
        while (plus * 2 <= n) plus *= 2;
        while (plus > 0) {
            if ((pos + plus <= n) && (tree[pos + plus] < x)) {
                x -= tree[pos + plus];
                pos += plus;
            }
            plus >>= 1;
        }
        return pos;
    }
};

void solve() {
    INT(n, k);
    STRING(S);
    STRING(T);
    int c = 0;
    for (auto s : S) {
        if (s == 'B')
            c++;
        else
            c--;
    }
    char s = c > 0 ? 'B' : 'C';
    vec(Pii, ans, 0);
    auto sw = [&](int i, int j) {
        ans.push_back({i + 1, j + 1});
#ifdef RIN__LOCAL
        fori(t, k) {
            swap(S[(i + t) % n], T[(j + t) % n]);
        }
#endif
    };

    vec(int, T_inds, 0);
    fori(i, n) {
        if (T[i] == s) T_inds.push_back(i);
    }
    BIT<ll> bit_T(2 * n + 10);

    vec(int, S_inds, 0);
    fori(i, n - 1, -1, -1) {
        if (S[i] != s) S_inds.push_back(i);
    }

    BIT<ll> bit_S(2 * n + 10);

    fori(p, len(T_inds)) {
        int sp = S_inds[p];
        sp += bit_S.sum(S_inds[p] + 1);
        bit_S.add(max<ll>(sp - k + 1, 0), 1);
        bit_S.add(sp + 1, -1);

        int ii = sp - k + 1;
        if (ii < 0) ii += n;
        if (false) {
            int i2 = -1;
            fori(i, n - 1, -1, -1) {
                if (S[i] != s) {
                    i2 = i;
                    break;
                }
            }
            i2 -= k - 1;
            if (i2 < 0) i2 += n;
            print("i", ii, i2);
        }

        int tp = T_inds[p];
        tp -= bit_T.sum(T_inds[p]);
        bit_T.add(tp, 1);
        bit_T.add(tp + k, -1);
        int jj = tp;

        if (false) {
            int j2 = -1;
            fori(j, n) {
                if (T[j] == s) {
                    j2 = j;
                    break;
                }
            }
            print("j", jj, j2);
        }

        int nj = jj + 1;
        if (nj == n) nj = 0;

        sw(ii, nj);
        sw(ii, jj);
    }

    print(len(ans));
    for (auto [i, j] : ans) {
        print(i, j);
    }

#ifdef RIN__LOCAL
    {
        print(S);
        sort(all(S));
        sort(all(T));
        assert(S[0] == S.back());
        assert(T[0] == T.back());
    }
#endif
}

int main() {
#ifndef INTERACTIVE
    cin.tie(0)->sync_with_stdio(0);
#endif
    cout << fixed << setprecision(12);
    int t;
    t = 1;
    // cin >> t;
    while (t--) solve();
    return 0;
}

// // #pragma GCC target("avx2")
// // #pragma GCC optimize("O3")
// // #pragma GCC optimize("unroll-loops")
// // #define INTERACTIVE
//
// #include "kyopro-cpp/template.hpp"
//
// #include "data_structure/BIT.hpp"
//
// void solve() {
//     INT(n, k);
//     STRING(S);
//     STRING(T);
//     int c = 0;
//     for (auto s : S) {
//         if (s == 'B')
//             c++;
//         else
//             c--;
//     }
//     char s = c > 0 ? 'B' : 'C';
//     vec(Pii, ans, 0);
//     auto sw = [&](int i, int j) {
//         ans.push_back({i + 1, j + 1});
// #ifdef RIN__LOCAL
//         fori(t, k) {
//             swap(S[(i + t) % n], T[(j + t) % n]);
//         }
// #endif
//     };
//
//     vec(int, T_inds, 0);
//     fori(i, n) {
//         if (T[i] == s) T_inds.push_back(i);
//     }
//     BIT<ll> bit_T(2 * n + 10);
//
//     vec(int, S_inds, 0);
//     fori(i, n - 1, -1, -1) {
//         if (S[i] != s) S_inds.push_back(i);
//     }
//
//     BIT<ll> bit_S(2 * n + 10);
//
//     fori(p, len(T_inds)) {
//         int sp = S_inds[p];
//         sp += bit_S.sum(S_inds[p] + 1);
//         bit_S.add(max<ll>(sp - k + 1, 0), 1);
//         bit_S.add(sp + 1, -1);
//
//         int ii = sp - k + 1;
//         if (ii < 0) ii += n;
//         if (false) {
//             int i2 = -1;
//             fori(i, n - 1, -1, -1) {
//                 if (S[i] != s) {
//                     i2 = i;
//                     break;
//                 }
//             }
//             i2 -= k - 1;
//             if (i2 < 0) i2 += n;
//             print("i", ii, i2);
//         }
//
//         int tp = T_inds[p];
//         tp -= bit_T.sum(T_inds[p]);
//         bit_T.add(tp, 1);
//         bit_T.add(tp + k, -1);
//         int jj = tp;
//
//         if (false) {
//             int j2 = -1;
//             fori(j, n) {
//                 if (T[j] == s) {
//                     j2 = j;
//                     break;
//                 }
//             }
//             print("j", jj, j2);
//         }
//
//         int nj = jj + 1;
//         if (nj == n) nj = 0;
//
//         sw(ii, nj);
//         sw(ii, jj);
//     }
//
//     print(len(ans));
//     for (auto [i, j] : ans) {
//         print(i, j);
//     }
//
// #ifdef RIN__LOCAL
//     {
//         print(S);
//         sort(all(S));
//         sort(all(T));
//         assert(S[0] == S.back());
//         assert(T[0] == T.back());
//     }
// #endif
// }
//
// int main() {
// #ifndef INTERACTIVE
//     cin.tie(0)->sync_with_stdio(0);
// #endif
//     cout << fixed << setprecision(12);
//     int t;
//     t = 1;
//     // cin >> t;
//     while (t--) solve();
//     return 0;
// }