#include<bits/stdc++.h>

using namespace std;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
typedef long long ll;
#define int ll
typedef unsigned long long ull;
typedef long double ld;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

#define pb push_back
#define all(x) x.begin(), x.end()
#define sz(x) (int)x.size()
#define mispertion ios_base::sync_with_stdio(0), cin.tie(0), cout.tie(0)
#define F first
#define S second
#define getlast(s) (*s.rbegin())
#define debg cout << "OK\n"

const ld PI = 3.1415926535;
const int N = 1e6+5;
const int M = 50 + 1;
const int mod = 1e9+7;
const int infi = 1e15;
const ll infl = LLONG_MAX;
const int P = 31;

int mult(int a, int b) {
    return a * 1LL * b % mod;
}

int sum(int a, int b) { 
    if (a + b < 0)
        return a + b + mod;
    if (a + b >= mod)
        return a + b - mod;
    return a + b;
}

ll binpow(ll a, ll n) {
    if (n == 0)
        return 1;
    if (n % 2 == 1) {
        return binpow(a, n - 1) * a % mod;
    } else {
        ll b = binpow(a, n / 2);
        return b * b % mod;
    }
}

int n, k, ans, ps[2 * N], R[2 * N], prp[2 * N], prc[2 * N];
string s;

void normalize(){
    vector<int> psa = {}, psb = {};
    for(int i = 1; i <= 2 * n; i++){
        if(s[i] == 'A')
            psa.pb(i);
        else
            psb.pb(i);
    }
    string ns = "#";
    int balance = 0, skp = 0, cur = 0;
    for(int i = 1; i <= 2 * n; i++){
        if(s[i] == 'A' && skp){
            skp--;
            continue;
        }
        if(s[i] == 'B' && balance == 0){
            ns += "A";
            ans += (psa[cur] - sz(ns) + 1);
            balance++;
            skp++;
            i--;
            cur++;
            continue;
        }
        if(s[i] == 'B'){
            ns += "B";
            balance--;
        }else{
            ns += "A";
            cur++;
            balance++;
        }
    }
    s = ns;
}

struct Line{
    ld k, b;
};

ld intersect(Line x, Line y){
    return (y.b - x.b) / (x.k - y.k);
}

struct convex{
    vector<pair<Line, int>> v;
    int cur = 0;
    
    void add(Line l, int a){
        while(sz(v) > 1 && make_pair(intersect(v[sz(v) - 2].F, v.back().F), v.back().S) >= make_pair(intersect(v.back().F, l), a)){
            if(sz(v) - 1 == cur)
                cur--;
            v.pop_back();
        }
        v.push_back({l, a});
    }

    pii get(int x){
        if(sz(v) == 0){
            return {infi, infi};
        }
        while(cur < sz(v) - 1 && intersect(v[cur].F, v[cur + 1].F) <= x)
            cur++;
        if(intersect(v[cur].F, v[cur - 1].F) == x){
            return make_pair(v[cur].F.k * x + v[cur].F.b, min(v[cur].S, v[cur - 1].S));
        }
        return make_pair(v[cur].F.k * x + v[cur].F.b, v[cur].S);
    }
};

pii getans(int C){
    vector<pair<int, int>> dp(n + 2, {infi, infi});
    dp[n + 1] = {0, 0};
    convex cht;
    int cj = n + 1;
    int l = n, r = n;
    while(l > 0 || r > 0){
        if(l == 0){
            int i = r + 1;
            if(i <= n){
                while(cj > 1 && R[i] < ps[cj - 1])
                    cj--;
                if(cj > i)
                    dp[i] = min(dp[i], {dp[cj].F, dp[cj].S + 1});
                dp[i].F += C;
            }
            ld b = dp[i].F + prc[ps[i - 1]] * ps[i - 1] - prp[ps[i - 1]] - (prc[ps[i - 1]] * prc[ps[i - 1]] - prc[ps[i - 1]]) / 2;
            ld k = ps[i - 1] - prc[ps[i - 1]];
            cht.add({k, b}, dp[i].S);
            r--;
        }else if(r == 0){
            int i = l;
            int nzm = prp[R[i] - 1] - (prc[R[i] - 1] * prc[R[i] - 1] + prc[R[i] - 1]) / 2;
            dp[i] = cht.get(-prc[R[i] - 1]);
            dp[i].S++;
            dp[i].F += nzm;
            l--;
        }else if(R[l] > ps[r]){
            int i = l;
            int nzm = prp[R[i] - 1] - (prc[R[i] - 1] * prc[R[i] - 1] + prc[R[i] - 1]) / 2;
            dp[i] = cht.get(-prc[R[i] - 1]);
            dp[i].S++;
            dp[i].F += nzm;
            l--;
        }else{
            int i = r + 1;
            if(i <= n){
                while(cj > 1 && R[i] < ps[cj - 1])
                    cj--;
                if(cj > i)
                    dp[i] = min(dp[i], {dp[cj].F, dp[cj].S + 1});
                dp[i].F += C;
            }
            ld b = dp[i].F + prc[ps[i - 1]] * ps[i - 1] - prp[ps[i - 1]] - (prc[ps[i - 1]] * prc[ps[i - 1]] - prc[ps[i - 1]]) / 2;
            ld k = ps[i - 1] - prc[ps[i - 1]];
            cht.add({k, b}, dp[i].S);
            r--;
        }
    }
    while(cj > 1 && R[1] < ps[cj - 1])
        cj--;
    if(cj > 1)
        dp[1] = min(dp[1], {dp[cj].F, dp[cj].S + 1});
    dp[1].F += C;
    return dp[1];
}

void solve(){
    cin >> n >> k;
    cin >> s;
    s = "#" + s;
    normalize();
    queue<int> st;
    for(int i = 1; i <= 2 * n; i++){
        prc[i] = prc[i - 1] + (s[i] == 'B');
        prp[i] = prp[i - 1] + (s[i] == 'B') * i;
    }
    int cnt = 0;
    for(int i = 2 * n; i >= 1; i--){
        if(s[i] == 'B'){
            st.push(i);
            continue;
        }
        R[n - cnt] = st.front();
        ps[n - cnt] = i;
        st.pop();
        cnt++;
    }
    int lo = -1, hi = 10 * n * n + 1;
    while(lo + 1 < hi){
        int C = (lo + hi) / 2;
        auto e = getans(C);
        if(e.S > k)
            lo = C;
        else
            hi = C;
    }
    int C = hi;
    auto e = getans(C);
    cout << e.F + ans - (k * C) << '\n';
}   

signed main() {
    mispertion;
    int t = 1;
    //cin >> t;
    while(t--){
        solve();
    }
    return 0;
}