#include <iostream>
#include <string>
#define N 1111111

using namespace std;

string s;

int n,a[37],l,r,k;
long long ans;

int f(char c){
    if (c>='0' && c<='9') return c-'0';
    return c-'a'+10;
}

int main(void){
    freopen("input.txt","r",stdin);
    freopen("output.txt","w",stdout);
    cin>>n>>k;
    cin>>s;
    r=-1;
    while (r<n-1){
        ++r;
        if (++a[f(s[r])]>k){
            while (l<r && a[f(s[r])]>k){
                --a[f(s[l])];
                ++l;
            }
        }
        ans+=r-l+1;
    }
    cout<<ans;
    return 0;
}