#include <bits/stdc++.h>
#define int long long
using namespace std;
const int N = 5e2 + 42, INF = 1e18 + 42;
vector<int> a, pre;
int n, k, x = 0, cumul[N], fi[N], dp[N][N];
signed main() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n >> k;
for(int i = 0; i < N; i++)
fi[i] = N-1;
for(int i = 0; i < 2*n; i++) {
char c; cin >> c;
if(c == 'A') {
if(fi[x] == N-1)
fi[x] = (int)a.size();
a.push_back(i);
pre.push_back(x);
} else {
x++;
}
}
for(int i = n-1; i > -1; i--)
if(fi[i] == N-1)
fi[i] = fi[i+1];
for(int i = 1; i <= n; i++)
cumul[i] = cumul[i-1] + pre[i-1];
for(int i = 1; i <= n; i++)
dp[0][i] = INF;
for(int nbG = 1; nbG <= k; nbG++) {
for(int i = 1; i <= n; i++) {
dp[nbG][i] = INF;
for(int j = 0; j < i; j++)
dp[nbG][i] = min(dp[nbG][i], dp[nbG-1][j] - (max(i, fi[j]) - max(j, fi[j])) * j + cumul[max(i, fi[j])] - cumul[max(j, fi[j])]);
}
}
cout << dp[k][n] << '\n';
}
