#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int inf = 1e9;
void solve () {
int n, k; cin >> n >> k;
string s; cin >> s;
int bad = 0;
int cur = 0;
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'B') {
cur--;
} else {
cur++;
}
if (cur < 0) {
int j = i;
while (s[j] != 'A') {
j++;
}
bad += j - i;
swap(s[i], s[j]);
i--; cur++;
}
}
int dp[n + 1][k + 1] = {};
vector <int> ii[2];
ii[0].push_back(0);
ii[1].push_back(0);
for (int i = 0; i < 2 * n; i++) {
ii[s[i] - 'A'].push_back(i + 1);
}
for (int i = 1; i <= n; i++) {
dp[i][0] = inf;
int l = i;
while (l > 1 && ii[1][l - 1] > ii[0][i]) {
l--;
}
for (int j = 1; j <= k; j++) {
dp[i][j] = dp[l - 1][j - 1];
int prv = ii[0][i];
int cur = 0;
for (int p = l - 1; p >= 1; p--) {
cur += prv - ii[1][p];
prv--;
dp[i][j] = min(dp[i][j], cur + dp[p - 1][j - 1]);
}
}
}
cout << bad + dp[n][k] << '\n';
}
signed main () {
ios::sync_with_stdio(0); cin.tie(0);
int tc = 1; //cin >> tc;
while (tc--) solve();
}
