#include <bits/stdc++.h>
#define MAX 300005
#define MOD (ll)(1048573)
#define INF (ll)(1e18)
#define inf (int)(1e9)
#define eps (double)(1e-8)
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
int tt, n, m, k, q;
string str;
int cnt[MAX];
ll dp[5005][5005];
ll cost(int i, int j) {
ll ret = 0;
for(int k = i; k <= j; ++k) {
ret += max(0, - (i - 1) + cnt[k]);
}
return ret;
}
int main() {
ios::sync_with_stdio(0);
cin.tie(NULL);
cin >> n >> k >> str;
for(int i = 0, tmp = 0, p = 0; i < 2 * n; ++i) {
if(str[i] == 'A') cnt[++p] = tmp;
else ++tmp;
}
for(int i = 0; i <= n; ++i) for(int j = 0; j <= k; ++j) dp[i][j] = INF;
dp[0][0] = 0;
for(int i = 1; i <= n; ++i) for(int j = 1; j <= k; ++j) {
for(int ii = 0; ii < i; ++ii) {
dp[i][j] = min(dp[i][j], dp[ii][j - 1] + cost(ii + 1, i));
}
}
cout << dp[n][k];
return 0;
}
