#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
#define int long long
#define fi first
#define se second
#define L(i, j, k) for (int i = (j); i <= (k); i++)
#define R(i, j, k) for (int i = (j); i >= (k); i--)
#define all(x) x.begin(), x.end()
const int inf = 1e18;
void solve() {
int n, req;
string s;
cin >> n >> req >> s;
vector<vector<int>> dp(n+1, vector<int> (n+1, inf));
vector<int> pre(n+1, inf);
int ca = 0, cb = 0;
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'A') ca++, pre[ca] = cb;
else if (s[i] == 'B') cb++;
}
dp[0][0] = 0;
for (int k = 0; k <= n; k++) {
for (int i = 0; i < n; i++) {
int cur = 0;
for (int j = 1; j <= n - i; j++) {
cur += max(0LL, pre[i + j] - i);
dp[i + j][k + 1] = min(dp[i + j][k + 1], dp[i][k] + cur);
}
}
}
cout << dp[n][req] << '\n';
}
int32_t main() {
ios::sync_with_stdio(0); cin.tie(0);
int test = 1;
// cin >> test;
while (test--) solve();
}
