#include <bits/stdc++.h>
using ll = long long;
using pii = std::pair<short, short>;
int main() {
  int n, k;
  std::cin >> n >> k;
  std::string s;
  std::cin >> s;
  std::map<char, int> map;
  for (auto ch : s) {
    map[ch]++;
  }
  std::map<int, std::set<char>> used;
  for (char c = 'a'; c <= 'z'; c++) {
    while (map[c] > 0) {
      int found = -1;
      for (int i = 0; i < k; i++) {
        if (!used[i].count(c)) {
          found = i;
          break;
        }
      }
      if (found == -1) {
        std::cout << "NO";
        return 0;
      }
      map[c]--;
      used[found].insert(c);
    }
  }
  std::cout << "YES";
  return 0;
}
