#include<bits/stdc++.h>
using namespace std;
typedef pair<int, int> pii;
typedef long long ll;

inline int rd() {
    int x = 0;
    bool f = 0;
    char c = getchar();
    for (; !isdigit(c); c = getchar()) f |= (c == '-');
    for (; isdigit(c); c = getchar()) x = x * 10 + (c ^ 48);
    return f ? -x : x;
}

#define N 500007

char s[N], t[N];

int sum[N], f[N];

deque<int> pos;

int main() {
    pos.push_back(0);
    int n = rd(), m = rd();
    scanf("%s", s + 1); scanf("%s", t + 1);
    for (int i = 1; i <= n; ++i) sum[i] = sum[i - 1] + (t[i] != t[i - 1]);

    // the number of segments in [l, r] : sum[r] - sum[l + 1] + 1
    auto val = [&](int p) {return 2 * f[p] - sum[p + 1];};
    for (int i = 1; i <= n; ++i) {
        if (s[i] == t[i]) f[i] = f[i - 1];
        else {
            while (i - pos.front() > m) pos.pop_front();
            f[i] = f[pos.front()] + (sum[i] - sum[pos.front() + 1] + 1) / 2 + 1;
        }
        while (!pos.empty() && val(pos.back()) >= val(i)) pos.pop_back();
        pos.push_back(i);
    }
    printf("%d\n", f[n]);
    return 0;
}