#include <bits/stdc++.h>
using namespace std;
int arr[30] = {};
int main() {
  ios::sync_with_stdio(0), ios_base::sync_with_stdio(0), cin.tie(0),
      cout.tie(0);
  ;
  int n, k;
  string s;
  cin >> n >> k >> s;
  for (int i = 0; i < n; ++i) arr[s[i] - 'a']++;
  for (int j = 1; j <= k; ++j)
    for (char i = 'a'; i <= 'z'; ++i)
      if (arr[(int)i - 'a']) arr[(int)i - 'a']--;
  for (int i = 0; i < 30; ++i)
    if (arr[i]) return cout << "NO\n", 0;
  cout << "YES\n";
  return 0;
}
