#include <bits/stdc++.h>   using namespace std;   bool ch[30];   int whenlast[30];   char g[1000005];   int main() {     int n, k;     cin >> n >> k;     for (int i = 0; i < n; i++) {       scanf(  %c , &g[i]);       whenlast[g[i] -  A ] = i;     }     int rlt = 0, ans = 0;     for (int i = 0; i < n; i++) {       ch[g[i] -  A ] = true;       rlt = 0;       for (int p = 0; p < 30; p++)         if (ch[p]) rlt++;       ans = max(ans, rlt);       for (int x = 0; x < 30; x++)         if (whenlast[x] == i) ch[x] = false;     }     if (ans <= k)       puts( NO );     else       puts( YES );     return 0;   }   