#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 500;
const ll INF = 1'000'000'000'000'000;
int n, k, a[N + 10], id[N + 10];
ll pre[N + 10][N + 10];
ll dp[N + 10][N + 10];
ll base;
void readInput() {
cin >> n >> k;
int x = 0, cntA = 0;
for (int i = 1; i <= n + n; i++) {
char c;
cin >> c;
if (c == 'B')
x++;
else {
id[++cntA] = min(n, x + 1);
a[min(n, x + 1)]++;
if (x == n)
base++;
}
}
}
void calcPre() {
for (int i = 1; i <= n; i++)
for (int j = i + 1; j <= n + 1; j++) {
pre[i][j] = pre[i][j - 1] + max(0, id[j - 1] - i);
}
}
void calcDP() {
for (int i = 2; i <= n + 1; i++) {
dp[i][0] = INF;
for (int j = 1; j <= n; j++) {
dp[i][j] = INF;
for (int x = 1; x < i; x++)
dp[i][j] = min(dp[i][j], dp[x][j - 1] + pre[x][i]);
}
}
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
readInput();
calcPre();
calcDP();
cout << dp[n + 1][k] + base << flush;
return 0;
}
