#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
//#include<bits/extc++.h>
//__gnu_pbds
const int N = 505;
ll dp[N][N];
int Bpa[N];
int main(){
ios_base::sync_with_stdio(0),cin.tie(0),cout.tie(0);
int n,K;cin>>n>>K;
string s;cin>>s;
int pa = 0;
int pb = 0;
for(int i=1;i<=2*n;i++){
if(s[i-1]=='A') pa++;
if(s[i-1]=='B') Bpa[++pb]=pa;
}
for(int i=1;i<=n;i++) dp[i][0]=1e15;
for(int k=1;k<=K;k++){
for(int i=1;i<=n;i++){
dp[i][k] = 1e18;
int cost = 0;
for(int j=1;j<=i;j++){
cost+=max(0,i-Bpa[i-j+1]);
dp[i][k] = min(dp[i][k],cost+dp[i-j][k-1]);
}
//cout<<dp[i][k]<<" ";
}
//cout<<"\n";
}
cout<<dp[n][K]<<"\n";
return 0;
}
