#include <bits/stdc++.h>
using namespace std;
#define sz(v) int(v.size())
#define ar array
typedef long long ll;
const int N = 5e3+10, MOD = 1e9+7;
int n, K, small[N];
int ps[N], l_use[N];
vector<int> one, two;
void solve() {
cin >> n >> K;
for (int i = 0; i < 2 * n; i++) {
char c; cin >> c;
if (c == 'A') one.push_back(i);
else two.push_back(i);
}
// for (int x : one) cerr << x << ' '; cerr << endl;
// for (int x : two) cerr << x << ' '; cerr << endl;
for (int i = 0; i < n; i++) {
small[i] = lower_bound(two.begin(), two.end(), one[i]) - two.begin();
}
for (int i = 0; i < n; i++) {
ps[i] = small[i] + (i ? ps[i-1] : 0);
l_use[i] = -(i ? ps[i-1] : 0);
for (int j = i; j < n; j++) {
if (small[j] < i) {
l_use[i] -= small[j] - i;
}
}
}
auto COST = [&](int l, int r) {
return ps[r] + l_use[l] - l * (r - l + 1);
};
/*
for (int l = 0; l < n; l++) {
int cur = 0;
for (int r = l; r < n; r++) {
cur += max(small[r] - l, 0);
cost[l][r] = cur;
}
}
for (int i = 0; i < n; i++) {
for (int j = i; j < n; j++) {
int one = cost[i][j];
int two = COST(i, j);
assert(one == 0 && two > 0 || one == two);
}
}
*/
vector<int> prv(n, MOD);
for (int k = 1; k <= K; k++) {
vector<int> nxt(n, MOD);
for (int i = 0; i < n; i++) {
for (int l = i; l >= 0; l--) {
nxt[i] = min(nxt[i], COST(l, i) + (l == 0 ? 0 : prv[l-1]));
}
int l = small[i];
if (l <= i) {
nxt[i] = min(nxt[i], (l == 0 ? 0 : prv[l-1]));
}
}
swap(prv, nxt);
}
cout << prv[n-1] << '\n';
}
int main() {
ios::sync_with_stdio(false); cin.tie(0);
int T = 1;
// cin >> T;
while (T--) solve();
}
