#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=1e6+5;
int n,k,t,c[N],g[N],fst[N],q[N];
ll sc[N],f[N],ans;
char s[N<<1];
ll Y(int i){return f[i]-sc[max(fst[i],i+1)-1]-1ll*i*(-max(fst[i],i+1)+1);}
double slope(int i,int j){
	return 1.0*(Y(i)-Y(j))/(i-j);
}
bool ok(ll v){
	int l=0,r=0;
	for(int i=1;i<=n;i++){
		while(l<r&&slope(q[l],q[l+1])<i) l++;
		int x=max(fst[q[l]],q[l]+1);
		f[i]=f[q[l]]+sc[i]-sc[x-1]-1ll*q[l]*(i-x+1)+v,g[i]=g[q[l]]+1;
		while(l<r&&slope(q[r-1],q[r])>=slope(q[r],i)) r--;
		q[++r]=i;
	}
	if(g[n]<=k) ans=f[n]-v*k;
	return g[n]<=k;
}
signed main(){
	scanf("%d%d%s",&n,&k,s+1);
	for(int i=1,b=0;i<=2*n;i++){
		if(s[i]=='A') c[++t]=b,sc[t]=sc[t-1]+c[t];
		else b++;
	}
	for(int i=1;i<=n;i++) fst[i]=lower_bound(c+1,c+1+t,i)-c;
	ll l=0,r=1e18;
	while(l<=r){
		ll mid=(l+r)/2;
		if(ok(mid)) r=mid-1;
		else l=mid+1;
	}
	printf("%lld\n",ans);
	return 0;
}