#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;
}
