#include <bits/stdc++.h>
using namespace std;
template <typename T>
void chmax(T &a, T b) {
  a = max(a, b);
}
template <typename T>
void chmin(T &a, T b) {
  a = min(a, b);
}
template <typename T>
void chadd(T &a, T b) {
  a = a + b;
}
long long int dx[] = {0, 0, 1, -1};
long long int dy[] = {1, -1, 0, 0};
constexpr long long INF = 1001001001001001LL;
constexpr long long MOD = 1000000007LL;
signed main() {
  long long int N, K;
  cin >> N >> K;
  string s;
  cin >> s;
  long long int cnt[30] = {};
  for (long long int(i) = (0); (i) < (s.length()); (i)++) {
    cnt[s[i] - 'a']++;
  }
  bool ok = true;
  for (long long int(i) = (0); (i) < (26); (i)++)
    if (cnt[i] > K) ok = false;
  cout << (ok ? "YES" : "NO") << endl;
  return 0;
}
