#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n, k;
cin >> n >> k;
string s;
cin >> s;
vector<int> a, b;
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'A') {
a.push_back(i);
} else {
b.push_back(i);
}
}
vector<int> pref(2 * n);
for (int i = 0; i < 2 * n; i++) {
if (i > 0) {
pref[i] = pref[i - 1];
}
pref[i] += (s[i] == 'A' ? 1 : 0);
}
auto Get = [&](int L, int R) {
return pref[R] - (L == 0 ? 0 : pref[L - 1]);
};
auto Solve = [&](int L, int R) {
int res = 0;
for (int i = L; i <= R; i++) {
if (b[i] < a[R]) {
res += Get(b[i], a[R]);
}
}
return res;
};
const int inf = (int) 1e9;
vector<int> dp(n + 1, inf);
dp[0] = 0;
for (int foo = 0; foo < k; foo++) {
vector<int> new_dp(n + 1, inf);
for (int i = 0; i < n; i++) {
for (int j = i; j < n; j++) {
new_dp[j + 1] = min(new_dp[j + 1], dp[i] + Solve(i, j));
}
}
swap(dp, new_dp);
}
cout << dp[n] << '\n';
return 0;
}
