#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int inf = 1e9;
int n, k, dp[5002][5002], t[5002], P[5002];
vector <int> ii[2];
string s;
int choose (int x) {
return x * (x - 1) / 2;
}
void solve () {
cin >> n >> k >> 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++;
}
}
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++) {
P[i] = P[i - 1] + ii[1][i];
}
for (int i = 1; i <= n; i++) {
dp[i][0] = inf;
}
for (int j = 1; j <= k; j++) {
int l = 1;
for (int i = 1; i <= n; i++) {
dp[i][j] = inf;
while (ii[1][l] < ii[0][i]) {
l++;
}
t[i] = l;
for (int p = t[i]; p >= 1; p--) {
dp[i][j] = min(dp[i][j], dp[p - 1][j - 1] + ii[0][i] * (t[i] - p) - choose(t[i] - p) - (P[t[i] - 1] - P[p - 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();
}
