#include <bits/stdc++.h>
inline long long MAX2(long long a, long long int b) {
  return (a) > (b) ? (a) : (b);
}
inline long long MAX3(long long a, long long b, long long c) {
  return (a) > (b) ? ((a) > (c) ? (a) : (c)) : ((b) > (c) ? (b) : (c));
}
inline long long MIN2(long long a, long long b) {
  return (a) < (b) ? (a) : (b);
}
inline long long MIN3(long long a, long long b, long long c) {
  return (a) < (b) ? ((a) < (c) ? (a) : (c)) : ((b) < (c) ? (b) : (c));
}
using namespace std;
int main() {
  int i, n, k, ar[27] = {};
  string in;
  cin >> n >> k;
  cin >> in;
  for (i = 0; i < n; i++) {
    ar[in[i] - 'a']++;
  }
  for (i = 0; i < 26; i++) {
    if (ar[i] > k) break;
  }
  if (i == 26)
    cout << "YES";
  else
    cout << "NO";
}
