#include <bits/stdc++.h>
    
using namespace std;
    
typedef long long ll;
const int  N = 5e2 + 12, MOD = (int)1e9 + 7;

int n, k, dp[N][N], c[N][N], f[N], pref[N];
string s;
vector<int> x, y;
const int inf = 1e9;

void prec() {
    auto get = [&](int l, int r) {
        int ret =0;
        for(int i = l; i <= r; i++) {
            if(y[i] > x[r]) continue;
            int t_ =0 ;
            t_ += pref[x[r]];
            if(y[i]> 0) t_ -= pref[y[i] - 1];
            ret += t_;

        }
        return ret;
    };
    for(int i = 1; i <= n; i++) {
        for(int j = i; j >= 1; j--) {
            c[j][i] = get(j, i);
        }
    }
}
int cost(int l, int r) {
    return c[l][r];
}
void test() {
    cin >> n >> k >> s;
    x.push_back(0);
    y.push_back(0);
    for(int i = 0; i < n + n; i++) {
        pref[i] = (s[i] == 'A');
        if(i > 0) pref[i] += pref[i - 1];
        if(s[i] == 'A') {
            x.push_back(i);
        } else {
            y.push_back(i);
        }
    }
    prec();
    for(int i = 1;i <= n; i++) {
        dp[i][0] = inf;
        for(int j = 1; j <= n; j++) {
            dp[i][j] = inf;
        }
        for(int j = 1; j <= i; j++) {
            for(int l = i; l >= 1; l--) {
                dp[i][j] = min(dp[i][j], dp[l - 1][j - 1] + cost(l, i));
            }
            // cout << dp[i][j] << ' ';
        }
        // cout << '\n';
    }
    cout << dp[n][k] << '\n';
}
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0); 
    
    int t = 1; 
    // cin >> t;
    
    while(t--) 
        test();
    return 0;
}