#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int MAXN=1e6+5;
const ll  inf=1e12;
int n,k,p[MAXN],c[MAXN],q[MAXN],f[MAXN];
ll s[MAXN],dp[MAXN];
char str[MAXN<<1];
inline ll y(int i) { return dp[i]-s[p[i]-1]+1ll*i*(p[i]-1); }
inline array<ll,2> check(ll x) {
	int hd=0,tl=0;
	for(int i=1;i<=n;++i) {
		while(hd<tl&&y(q[hd+1])-y(q[hd])<1ll*(q[hd+1]-q[hd])*i) ++hd;
		int j=q[hd]; dp[i]=dp[j]+s[i]-s[p[j]-1]-1ll*j*(i-p[j]+1)+x,f[i]=f[j]+1;
		while(hd<tl&&(y(q[tl])-y(q[tl-1]))*(i-q[tl])>(y(i)-y(q[tl]))*(q[tl]-q[tl-1])) --tl;
		q[++tl]=i;
	}
	return {f[n],dp[n]};
}
signed main() {
	scanf("%d%d%s",&n,&k,str+1);
	for(int i=1,id=0,cnt=0;i<=2*n;++i) {
		if(str[i]=='A') c[++id]=cnt,s[id]=c[id]+s[id-1];
		else ++cnt;
	}
	for(int i=1;i<=n;++i) p[i]=lower_bound(c+1,c+n+1,i)-c,p[i]=max(p[i],i+1);
	ll l=0,r=inf,res=inf;
	while(l<=r) {
		ll mid=(l+r)>>1;
		if(check(mid)[0]<=k) res=mid,r=mid-1;
		else l=mid+1;
	}
	printf("%lld\n",check(res)[1]-k*res);
	return 0;
}