#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define all(a) a.begin(),a.end()
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
typedef unsigned long long ull;
typedef vector<int> vi;
int dp[1001][501];
vector<int> A(1,0), B(1,0);
int Cost(int j, int i){
int len = i-j, res = 0;
for(int l = 1; l <= len/2; l++){
res += max(0,A[l+j/2] - j-l);
}
return res;
}
void solve(){
int n,k; cin >> n >> k;
for(int i = 1; i <= 2*n; i++){
char c; cin >> c;
if(c=='A') A.emplace_back(i);
else B.emplace_back(i);
}
for(int i = 2; i <= 2*n; i+=2){
fill(dp[i],dp[i]+k+1,1e9);
for(int j = 0; j < i; j+=2){
int c = Cost(j,i);
for(int l = 1; l <= k; l++)
dp[i][l] = min(dp[i][l],dp[j][l-1]+c);
}
}
int ans = 1e9;
for(int j = 1; j <= k; j++) ans = min(ans,dp[2*n][j]);
cout << ans;
}
signed main(){
cin.tie(0)->sync_with_stdio(0);
int t = 1;
// cin >> t;
while(t--){solve();}
}
