#include <bits/stdc++.h>
using namespace std;
const long long BASE = (long long)1e9 + 7;
inline long long gcd(long long a, long long b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
inline long long lcm(long long a, long long b) { return a / gcd(a, b) * b; };
inline long long fpow(long long a, long long n, long long p = BASE) {
  long long r = 1;
  for (; n; n >>= 1) {
    if (n & 1) r = r * a % p;
    a = a * a % p;
  }
  return r;
}
inline long long inverse(long long a, long long p) { return fpow(a, p - 2, p); }
inline long long addmod(long long &a, long long val, long long p = BASE) {
  if ((a = (a + val)) > p) a -= p;
}
inline long long submod(long long &a, long long val, long long p = BASE) {
  if ((a = (a - p)) < 0) a += p;
}
inline long long multi(long long &a, long long b, long long p = BASE) {
  return (long long)a * b % BASE;
}
void Solves();
long long MARK[256];
int main() {
  Solves();
  return 0;
}
void Solves() {
  long long n, k;
  cin >> n >> k;
  string s;
  cin >> s;
  bool flag = true;
  for (long long i = (0); i <= (n - 1); i++) {
    MARK[s[i]]++;
  }
  long long cnt = 0;
  for (long long i = (0); i <= (n - 1); i++) {
    if (MARK[s[i]] > k) {
      flag = false;
      break;
    }
  }
  if (flag)
    cout << "YES";
  else
    cout << "NO";
}
