/*
#pragma GCC optimize("Ofast,unroll-loops")
#pragma GCC target("avx2,fma,bmi,bmi2,sse4.2,popcnt,lzcnt")
*/
#include <bits/stdc++.h>
#define taskname ""
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define i64 long long
#define i128 __int128
#define pb push_back
#define ff first
#define ss second
#define isz(x) (int)x.size()
using namespace std;
const int mxN = 2e5 + 5;
const int mod = 1e9 + 7;
const i64 oo = 1e18;
int n, k;
int pfx[505], dp[505][505], min_swap[505][505];
string s;
vector<int> posA, posB;
int sum_range(int l, int r) {
if (l > r) return 0;
return (l + r) * (r - l + 1) / 2;
}
int calcA(int l, int r) {
if (l > r) return 0;
return (l == 0 ? pfx[r] : pfx[r] - pfx[l - 1]);
}
void upd(int &val, int nval) {
val = (val == -1 ? nval : (val < nval ? val : nval));
}
void solve() {
cin >> n >> k >> s; s = " " + s;
if (n > 500) return;
for (int i = 1; i <= 2 * n; ++i) {
if (s[i] == 'A') posA.emplace_back(i);
else posB.emplace_back(i);
}
partial_sum(all(posA), pfx);
for (int l = 0; l < n; ++l) {
for (int r = l; r < n; ++r) {
int pos = lower_bound(posA.begin() + l, posA.begin() + r + 1, posB[l]) - posA.begin();
int offset = 0;
if (pos <= r) offset = count(s.begin() + posB[l], s.begin() + posA[pos], 'A');
min_swap[l + 1][r + 1] = calcA(pos, r) - sum_range(posB[l] + offset, posB[l] + offset + (r - pos + 1) - 1);
// cout << l + 1 << " " << r + 1 << " " << min_swap[l + 1][r + 1] << endl;
}
// cout << "\n";
}
memset(dp, -1, sizeof dp);
dp[0][0] = 0;
for (int j = 0; j < k; ++j) {
for (int i = 0; i < n; ++i) if (dp[i][j] != -1) {
for (int ni = i + 1; ni <= n; ++ni) {
upd(dp[ni][j + 1], dp[i][j] + min_swap[i + 1][ni]);
}
}
}
cout << dp[n][k] << endl;
}
signed main() {
#ifndef CDuongg
if(fopen(taskname".inp", "r"))
assert(freopen(taskname".inp", "r", stdin)), assert(freopen(taskname".out", "w", stdout));
#else
freopen("bai3.inp", "r", stdin);
freopen("bai3.out", "w", stdout);
auto start = chrono::high_resolution_clock::now();
#endif
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
int t = 1; //cin >> t;
while(t--) solve();
#ifdef CDuongg
auto end = chrono::high_resolution_clock::now();
cout << "\n"; for(int i = 1; i <= 100; ++i) cout << '=';
cout << "\nExecution time: " << chrono::duration_cast<chrono::milliseconds> (end - start).count() << "[ms]" << endl;
cout << "Check array size pls sir" << endl;
#endif
}
