#include <bits/stdc++.h>
//#pragma GCC optimize("O3")
//#pragma GCC target("avx,avx2,fma")
#define sz(x) int((x).size())
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
using namespace std;
#define int ll
using ll = long long;
using ld = long double; // or double, if TL is tight
using str = string;
using ii = pair<int, int>;
using pl = pair<ll, ll>;
using vi = vector<int>;
using vll = vector<ll>;
signed main() {
cin.tie(0);
ios_base::sync_with_stdio(0);
int n, k;
string s;
cin >> n >> k >> s;
int nrB = 0;
vi A;
for(int i = 0; i < 2 * n; ++i) {
if(s[i] == 'B') ++nrB;
else A.push_back(nrB);
}
const int INF = 1e9;
vector<vi> Cost(n, vi(n, 0));
for(int i = 0; i < n; ++i) {
for(int j = i; j < n; ++j) {
Cost[i][j] = max(0ll, A[j] - i);
if(j) Cost[i][j] += Cost[i][j - 1];
}
}
vector<vi> DP(n, vi(k + 1, INF));
for(int i = 0; i < n; ++i) {
for(int j = 1; j <= k; ++j) {
if(j == 1) DP[i][j] = Cost[0][i];
for(int ant = 1; ant <= i; ++ant) {
DP[i][j] = min(DP[i][j], DP[ant - 1][j - 1] + Cost[ant][i]);
}
}
}
cout << DP[n - 1][k] << "\n";
return 0;
}
