#include <iostream>
#include <bits/stdc++.h>
#define ll long long
using namespace std;
const ll maxn = 2*1e5+5, INF = 4e18+9;
void solve(){
int N, K;
cin >> N >> K;
string s;
cin >> s;
s = ' ' + s;
int j = 1;
bool skip = false;
ll EX = 0;
for(int i = 1; i <= N*2; i++){
if(skip){
skip = false;
continue;
}
if(s[i] == 'A') continue;
while(s[j] == 'B'){
j++;
}
assert(j <= N*2);
if(j > i){
EX += j-i;
swap(s[i], s[j]);
skip = true;
}
j++;
}
int it = -1;
ll poss = 0, owe = 0, cur = 0;
vector<ll> sump(1), X(1), cnt(1);
for(int i = 1; i <= N*2; i++){
if(s[i] == 'A'){
cur++;
it++;
poss += i - it;
}else{
if(owe > 0){
owe--;
}else{
sump.push_back(poss);
X.push_back(i);
cnt.push_back(cur);
owe = cur-1;
cur = 0;
poss = 0;
}
}
}
int n = X.size()-1;
vector<ll> pfsp(n+1, 0), pfcnt(n+1, 0);
for(int i = 1; i <= n; i++){
pfsp[i] = pfsp[i-1] + sump[i];
pfcnt[i] = pfcnt[i-1] + cnt[i];
}
//lets do subtask n <= 500 first!
if(n <= K){
cout << EX;
return;
}
vector<vector<ll>> f(n+1, vector<ll>(K+1, INF));
f[0][0] = 0;
for(int i = 1; i <= n; i++){
for(int j = 1; j <= K; j++){
for(int p = 1; p <= i; p++){
ll cost = pfsp[i]-pfsp[p] - (pfcnt[i]-pfcnt[p])*X[p] + pfcnt[p]*(pfcnt[i]-pfcnt[p]);
f[i][j] = min(f[i][j], f[p-1][j-1] + cost);
}
}
}
cout << f[n][K] + EX;
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
solve();
}
