#include<bits/stdc++.h>
using namespace std;
using ll=long long;
using pii=pair<int,int>;
#define f first
#define s second
const ll inf=1e18;
int a[1000005],b[1000005],pos[1000005],pos2[1000005];
ll dp[2][1000005],qs[1000005];
ll f(int L,int R){
int x=pos2[L];
if(x>R) return 0;
return qs[R]-qs[x-1]-L*(R-x+1);
}
void play2(int n,bool z){
for(int i=1;i<=n;++i){
dp[z][i]=inf;
for(int j=0;j<i;++j){
if(pos2[j]>i){
dp[z][i]=min(dp[z][i],dp[!z][j]);
break;
}
dp[z][i]=min(dp[z][i],-j*i+dp[!z][j]-qs[pos2[j+1]-1]+1LL*pos2[j+1]*(j+1)-j+qs[i]-i-1);
}
}
}
void play(int l,int r,int optl,int optr,int z){
if(l>r) return;
int mid=l+r>>1;
pair<ll,int> best(inf,0);
for(int i=optl;i<=min(mid,optr);++i){
best=min(best,{dp[!z][i-1]+f(i,mid),i});
}
dp[z][mid]=best.f;
play(l,mid-1,optl,best.s,z);
play(mid+1,r,best.s,optr,z);
}
int main(){
ios::sync_with_stdio(false); cin.tie(0);
int n,K; cin>>n>>K;
string s; cin>>s;
for(int i=0,ia=0,ib=0;i<s.length();++i){
if(s[i]=='A') a[++ia]=i;
else b[++ib]=i;
}
for(int i=1,j=1;i<=n;++i){
while(j<=n&&b[j]<a[i]) ++j;
pos[i]=j;
qs[i]=qs[i-1]+j;
}
for(int i=1,j=1;i<=n;++i){
while(j<=n&&pos[j]<=i) ++j;
j=max(i,j);
pos2[i]=j;
}
for(int i=1;i<=n;++i) dp[1][i]=qs[i]-i;
for(int k=2;k<=K;++k){
// play(1,n,1,n,k&1);
play2(n,k&1);
}
cout<<dp[K&1][n]<<endl;
return 0;
}
