#include <bits/stdc++.h>
using namespace std;
bool prime[10000005];
long long int gcd(long long int a, long long int b) {
  if (a == 0) return b;
  return gcd(b % a, a);
}
void sieve() {
  int n = 10000005;
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (int i = p * 2; i <= n; i += p) prime[i] = false;
    }
  }
}
double sq(double a) { return a * a; }
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  long long int i, j, k, l, n, m, cnt = 0, sum = 0, t;
  long long int arr[1005][5];
  char a[1000005];
  cin >> n >> k;
  int hah[30] = {0};
  i = 0;
  cin >> a;
  while (a[i]) {
    hah[a[i++] - 'a']++;
  }
  for (i = 0; i < 26; i++) {
    if (hah[i] > k) break;
  }
  if (i < 26)
    cout << "NO";
  else
    cout << "YES";
}
