#include <algorithm>
#include <array>
#include <bitset>
#include <cassert>
#include <chrono>
#include <climits>
#include <cmath>
#include <complex>
#include <cstring>
#include <functional>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <set>
#include <vector>
#include <stack>
using namespace std;
#define int long long
#define mp make_pair
#define fi first
#define pii pair<int,int>
#define se second
const int INF=1000000000000000000;
//const int INF=1e9;
const int N=1000000;
//const int M=998244353;
const int ln=20;
template<typename T>
std::ostream& operator<< (std::ostream& os,pair<T,T> p){
return os << p.fi << "," << p.se << " ";
}
void solve(){
int n,k;
cin >> n >> k;
string s;
cin >> s;
if(n>500) return;
vector<int> a;
int cnt=0;
for(int i=0;i<2*n;i++){
if(s[i]=='A'){
cnt++;
} else a.push_back(cnt);
}
//for(int i=0;i<n;i++) cout << a[i] << " ";
cout << "\n\n";
int prefix[n+1][n+1];
for(int i=0;i<=n;i++) prefix[i][0]=0;
for(int j=0;j<=n;j++) for(int i=0;i<n;i++) prefix[j][i+1]=prefix[j][i]+max(j-a[i],0LL);
int dp[n+1][k+1];
for(int i=0;i<=k;i++) dp[0][i]=0;
for(int i=1;i<=n;i++) dp[i][0]=INF;
int opt[n][k];
for(int j=0;j<k;j++){
for(int i=0;i<n;i++){
// we want dp[i+1][j+1];
int ans=INF;
opt[i][j]=-1;
for(int m=0;m<=i;m++){
int cans=dp[m][j]+prefix[i+1][i+1]-prefix[i+1][m];
if(ans>cans){
opt[i][j]=m;
ans=cans;
}
}
dp[i+1][j+1]=ans;
}
}
for(int i=0;i<n;i++){
for(int j=0;j<k-1;j++){
assert(opt[i][j]<=opt[i][j+1]);
}
}
for(int i=0;i<n-1;i++){
for(int j=0;j<k;j++){
assert(opt[i][j]<=opt[i+1][j]);
}
}
cout << dp[n][k];
}
signed main(){
auto begin = std::chrono::high_resolution_clock::now();
ios_base::sync_with_stdio(false);
cin.tie(NULL); cout.tie(NULL);
int t;
//cin >> t;
t=1;
while(t--) solve();
auto end = std::chrono::high_resolution_clock::now();
auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin);
cerr << "Time measured: " << elapsed.count() * 1e-9 << " seconds.\n";
}
