#include<bits/stdc++.h>
#define Gc() getchar() 
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define PB push_back
#define fi first
#define se second
using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;
using namespace std;const int N=1e6+5,M=5e6+5,K=3e7+5,mod=998244353,Mod=mod-1;const db eps=1e-9;const int INF=1e9+7;mt19937 rnd(time(0));
int n,k,A[N],m,To[N],Q[N],H,T,g[N];ll Sum[N],f[N];char s[N];
ll YY(int x){return 1ll*To[x]*x-Sum[To[x]]+f[x];}
db slope(int x,int y){return (YY(x)-YY(y))*1.0/(x-y);}
int CK(ll mid){
	int R=0,j,h;H=1;T=0;Me(f,0x3f);Me(g,0x3f);f[0]=0;g[0]=0;
    for(j=1;j<=n;j++){
		if(A[j]<j) f[j]=f[A[j]]+mid,g[j]=g[A[j]]+1;
		while(R<j&&To[R]<=j) {
			while(H<T&&slope(Q[T-1],Q[T])>slope(Q[T],R)) T--;Q[++T]=R++;
		}
		while(H<T&&slope(Q[H],Q[H+1])<j) H++;
		// cerr<<H<<' '<<R<<'\n';
		if(H<=T){
			h=Q[H];ll w=f[h]+Sum[j]-Sum[To[h]]-1ll*(j-To[h])*h+mid;
			if(f[j]>w) f[j]=w,g[j]=g[h]+1;
			else if(f[j]==w&&g[j]>g[h]+1) g[j]=g[h]+1;
		} 
		// cerr<<f[j]<<'\n';
	}//cerr<<mid<<' '<<g[n]<<' '<<f[n]<<'\n';
	return g[n]<=k;
}
int main(){
    // freopen("1.in","r",stdin);
    int i,j,h;scanf("%d%d",&n,&k);scanf("%s",s+1);m=1;for(i=1;i<=2*n;i++) if(s[i]=='A') m++,A[m]=A[m-1];else A[m]++;
    Me(f,0x3f);f[0]=0;for(i=1;i<=n;i++) Sum[i]=Sum[i-1]+A[i];A[n+1]=INF;for(i=0;i<=n;i++) for(j=i+1;j<=n+1;j++) if(A[j]>i) {To[i]=j-1;break;}
	// for(i=0;i<=n;i++) cerr<<To[i]<<' ';cerr<<'\n';
	ll l=-1,r=1e12,mid;while(l+1<r) mid=l+r>>1,(CK(mid)?r:l)=mid;CK(r);cerr<<r<<'\n';
	printf("%lld\n",f[n]-k*r);
}