#include <bits/stdc++.h>
    
using namespace std;
    
typedef long long ll;
const int  N = 5e3 + 12, MOD = (int)1e9 + 7;

#define int ll
int n, k, x[N], y[N], pref[ 2 * N], col[N];
ll dp[N], c[N][N];
void prec() {
    for(int i = 1; i <= n; i++) {
        for(int j = i; j >= 1; j--) {
            c[j][i] = c[j + 1][i];
            if(y[j] <= x[i]) {
                c[j][i] += pref[x[i]] - pref[y[j] - 1];;
            }
        }
    }
}
pair<ll, ll> calc(ll pen) {
    for(int i = 1; i <= n; i++) {
        dp[i] = 1e18;
        col[i] = n + 1;
        for(int j = i; j >= 1; j--) {   
            if(make_pair(dp[j - 1] + c[j][i] - pen, -(col[j - 1] + 1)) < make_pair(dp[i], - col[i])) {
                dp[i] = dp[j - 1] + c[j][i] - pen;
                col[i] = col[j - 1] + 1;
            }
        }
    }
    return {dp[n], col[n]};
}
void test() {
    cin >> n >> k;
    int _x = 0, _y = 0;
    for(int i = 1; i <= n + n; i++) {
        char a;
        cin >> a;
        pref[i] = pref[i - 1];
        if(a == 'A') {
            x[++_x] = i;
            pref[i]++;
        } else { 
            y[++_y] = i;
        }
    }
    prec();
    // for(int i = -100; i <= 100; i++) {
        // auto [val, t] = calc(i);
        // cout << i << ' ' << val << ' ' << t << '\n';
    // }
    // return;
    ll l = -1e12, r = 1e12, res;
    while(r - l > 1) { 
        ll mid = (l + r) / 2;
        auto [val, t] = calc(mid);
        // cout << mid << ' ' << val << ' ' << t << '\n';
        if(t >= k) {
            r = mid;
            res = val + k * 1ll * mid;
        } else {
            l = mid;
        }
    }
    cout << res << '\n';
}
signed main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0); 
    
    int t = 1; 
    // cin >> t;
    
    while(t--) 
        test();
    return 0;
}