#include <bits/stdc++.h>
using namespace std;
char a[1000005];
pair<int, int> t[26];
int ans = -1000000000, x, k;
int main() {
  scanf("%d %d", &x, &k);
  scanf("%s", a);
  for (int i = 0; i < 26; i++) {
    t[i].first = -1;
    t[i].second = -1;
  }
  for (int i = 0; i < x; i++) {
    if (t[(int)a[i] - 65].first == -1) t[(int)a[i] - 65].first = i;
    if (t[(int)a[i] - 65].second < i) t[(int)a[i] - 65].second = i;
  }
  for (int i = 0; i < x; i++) {
    int cnt = 0;
    for (int j = 0; j < 26; j++)
      if (t[j].first != -1 && i >= t[j].first && i <= t[j].second) cnt++;
    ans = max(ans, cnt);
  }
  if (ans > k)
    printf("YES\n");
  else
    printf("NO\n");
  return 0;
}
