#include <bits/stdc++.h>
using namespace std;
int hh,tt;//开头，结尾
queue<char>s;
bool check(char ch){
for(int i = hh;i<=tt;i++){
if(s.front()+i==ch)return true;
}
return false;
}
int main(){
int n,m,sum=0;
cin >> n >> m;
for(int i = 1;i<=n;i++){
char ch;
cin >> ch;
if(check(ch)==false){
sum++;
s.push(ch);
if(tt-hh>m)s.pop();
}
}
