#include<bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<ll,int> pli;
#define fir first
#define sec second
const int maxn=1e6+10;
const ll inf=1e12;
const ll llinf=0x3f3f3f3f3f3f3f3f;

inline int read(){
    int x=0,w=1;char c=getchar();
    while(c<'0'||c>'9'){if(c=='-')w=-1;c=getchar();}
    while(c<='9'&&c>='0'){x=(x<<3)+(x<<1)+c-'0';c=getchar();}
    return x*w;
}

int n,m;
char s[maxn<<1];
int pre[maxn],pos[maxn];
ll sum[maxn];
int q1[maxn],head1,tail1;
int q2[maxn],head2,tail2;
int q3[maxn],head3,tail3;
pli f[maxn];
inline ll X(int A){
    return A;
}
inline ll Y(int A){
   return f[A].fir-sum[pos[A]-1]+1ll*(pos[A]-1)*A; 
}
inline ll w(int j,int i){
    return sum[i]-sum[pos[j]-1]-1ll*(i-pos[j]+1)*j;
}
inline bool check_slope1(int A,int B,int K){
    if((__int128_t)(Y(B)-Y(A))==(__int128_t)K*(X(B)-X(A))) return f[A].sec<=f[B].sec;
    else return (__int128_t)(Y(B)-Y(A))<(__int128_t)K*(X(B)-X(A));
}
inline bool check_slope2(int A,int B,int C){
    if((__int128_t)(Y(B)-Y(A))*(X(C)-X(B))==(__int128_t)(Y(C)-Y(B))*(X(B)-X(A))) return f[A].sec>=f[B].sec;
    else return (__int128_t)(Y(B)-Y(A))*(X(C)-X(B))>(__int128_t)(Y(C)-Y(B))*(X(B)-X(A));
}
inline void calc(ll k){
    f[0]=make_pair(0,0);
    q1[1]=0,head1=1,tail1=1;
    head2=1,tail2=0,head3=1,tail3=0;
    for(int i=1,j;i<=n;++i){
        f[i]=make_pair(llinf,0);
        while(head2<=tail2&&pos[q2[head2]]<=i){
            j=q2[head2++];
            while(head1<tail1&&check_slope2(q1[tail1-1],q1[tail1],j)) --tail1;
            q1[++tail1]=j;
        }
        while(head3<=tail3&&pos[q3[head3]]<=i) ++head3;
        if(head3<=tail3){
            j=q3[head3];
            f[i].fir=f[j].fir+k,f[i].sec=f[j].sec+1;
        }
        while(head1<tail1&&check_slope1(q1[head1],q1[head1+1],i)) ++head1;
        j=q1[head1];
        pli now=make_pair(f[j].fir+w(j,i)+k,f[j].sec+1);
        if(f[i].fir>now.fir||(f[i].fir==now.fir&&f[i].sec<now.sec)) f[i]=now;
        q2[++tail2]=i;
        while(head3<=tail3&&(f[i].fir<f[q3[tail3]].fir||(f[i].fir==f[q3[tail3]].fir&&f[i].sec>f[q3[tail3]].sec))) --tail3;
        q3[++tail3]=i;
    }
}
ll ans1,ans2;

int main(){
    n=read(),m=read();
    scanf("%s",s+1);
    for(int i=1,k=0,p=0;i<=2*n;++i){
        if(s[i]=='A'){
            if(k>=0){
                s[++p]='A',ans1+=i-p;
            }
            else{
                s[++p]='A',ans1+=i-p;
                s[++p]='B';
            }
            ++k;
        }
        else{
            if(k>0) s[++p]='B';
            --k;
        }
    }
    for(int i=0;i<=n;++i) pos[i]=n+1;
    for(int i=1,j=0,ca=0,cb=0;i<=2*n;++i){
        if(s[i]=='A'){
            ++ca;
            while(j<=cb) pos[j++]=ca;
            pre[ca]=cb;
        }
        else ++cb;
    }
    for(int i=1;i<=n;++i) sum[i]=sum[i-1]+pre[i];
    ll L=-inf,R=inf;
    while(L<=R){
        ll mid=(L+R)>>1;
        calc(mid);
        if(f[n].sec<m) R=mid-1;
        else ans2=f[n].fir-m*mid,L=mid+1;
    }
    printf("%lld\n",ans1+ans2);
    return 0;
}