#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
void solve () {
int n, k; cin >> n >> k;
string s; cin >> s;
int bad = 0;
while (true) {
bool flag = 0;
int cur = 0;
for (int i = 0; i < 2 * n; i++) {
if (s[i] == 'B') {
cur--;
} else {
cur++;
}
if (cur < 0) {
bad++;
while (s[i + 1] != 'A') {
i++;
}
swap(s[i], s[i + 1]);
flag = 1;
break;
}
}
if (!flag) {
break;
}
}
int dp[n + 1][k + 1] = {};
vector <int> ii[2];
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++) {
dp[i][0] = 1e9;
for (int j = 1; j <= k; j++) {
dp[i][j] = 1e9;
int l = i;
while (l > 1 && ii[1][l - 1] > ii[0][i]) {
l--;
}
dp[i][j] = min(dp[i][j], dp[l - 1][j - 1]);
auto g = ii[0], h = ii[1];
int cur = 0;
for (int p = l - 1; p >= 1; p--) {
while (h[p] < g[i]) {
cur++; h[p]++;
for (auto &x : g) {
if (x == h[p]) {
x--;
break;
}
}
}
dp[i][j] = min(dp[i][j], cur + dp[p - 1][j - 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();
}
