#include <bits/stdc++.h>
const int maxn = 100010;
const double EPS = 1e-6;
using namespace std;
int v[26];
int main() {
  int n, m;
  cin >> n >> m;
  string s;
  cin >> s;
  for (int i = 0; i < (s.size()); ++i) v[s[i] - 'a']++;
  int flag = 1;
  for (int i = 0; i < (26); ++i) {
    if (v[i] > m) flag = 0;
  }
  if (flag)
    cout << "YES" << endl;
  else
    cout << "NO" << endl;
  return 0;
}
