#include<bits/stdc++.h>
#define pb push_back
#define P make_pair
#define fi first
#define se second
#define bit(s,x) (((s)>>(x))&1)
#define pnp(s) __builtin_popcountll(s)
#define rep(i,a,b) for(int i=(a);i<=(b);++i)
#define per(i,a,b) for(int i=(a);i>=(b);--i)
using namespace std;
typedef long long ll; 
typedef unsigned long long ull;
typedef pair<int,int> pii;
inline ll read(){
	ll x=0,f=1,c=getchar();
	while(c<'0'||c>'9')f=(c=='-'?-1:1),c=getchar();
	while(c>='0'&&c<='9')x=(x<<1)+(x<<3)+(c^48),c=getchar();
	return x*f;
}
const int N=1e6+3;
int n,K,b[N],g[N];
char str[N<<1];
ll f[N],c[N];
inline ll Y(int x){return f[x]+1ll*x*b[x]-c[b[x]];}
inline void calc(ll x){
	static int dq[N];int hh=1,tt=1;dq[1]=0;
	rep(i,1,n){
		while(hh<tt && Y(dq[hh+1])-Y(dq[hh])<1ll*(dq[hh+1]-dq[hh])*i)++hh;
		int j=dq[hh];
		f[i]=Y(j)-1ll*j*i+c[i]-x;
		g[i]=g[j]+1;
		while(hh<tt && (Y(i)-Y(dq[tt-1]))*(dq[tt]-dq[tt-1])<(Y(dq[tt])-Y(dq[tt-1]))*(i-dq[tt-1]))--tt;
		dq[++tt]=i;
	}
}
int main() {
	scanf("%d%d%s",&n,&K,str+1);
	for(int i=1,j=0;i<=2*n;++i) if(str[i]=='A')++j,c[j]=i-j;
	rep(i,1,n){
		b[i]=b[i-1];
		while(b[i]<n&&c[b[i]+1]<i)++b[i];
	}
	//rep(i,1,n){
		//assert(c[b[i]]<i);
		//if(b[i]<n)assert(c[b[i]+1]>=i);
	//}
	rep(i,1,n)c[i]+=c[i-1],b[i]=max(b[i],i);
	ll l=-5e11,r=0;
	while(l<=r){
		ll m=(l+r)>>1;
		calc(m);
		if(g[n]<=K)l=m+1;
		else r=m-1;
	}
	calc(r);
	cout<<f[n]+r*K;
    return 0;
}