#include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef pair<int, int> pii;
typedef vector<pii> vpii;
#define sz(x) (int)(x).size()
#define all(x) (x).begin(), (x).end()
const ll INF = 1LL << 60;
int N, K;
vi S;
int cnt[2];
vi pos[2];
vvi dp;
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
cin >> N >> K;
string SS;
cin >> SS;
S.resize(2 * N);
for (int foo = 0; foo < 2; ++foo)
pos[foo].assign(N, -1);
for (int i = 0; i < 2 * N; ++i)
{
S[i] = (SS[i] == 'B');
pos[S[i]][cnt[S[i]]++] = i;
}
dp = vvi(N + 1, vi(K + 1, INF));
dp[0][0] = 0;
dp[0][1] = 0;
for (int i = 0; i < N; ++i)
dp[i + 1][1] = dp[i][1] + pos[0][i] - i;
for (int k = 1; k < K; ++k)
{
// compute k + 1 with k and iterative approach
for (int i = k; i <= N; ++i)
{
// working from base i times 0 and i times 1;
ll tmp = dp[i][k];
int border = 2 * i;
for (int j = i; j < N; ++j)
{
if (pos[0][j] < border)
++border;
else
tmp += pos[0][j] - border++;
dp[j + 1][k + 1] = min(dp[j + 1][k + 1], tmp);
}
}
}
cout << dp[N][K] << endl;
return 0;
}
