#include <bits/stdc++.h>
using namespace std;
int z, mn, mx, sum, ans, k, pos, pos1, r, xx, yy, zz, n, m, x, y;
vector<int> v;
bool OK[1000002], KO[1000002], zoom;
int b[100003], c[100002];
map<char, int> mp, mr;
set<int> st;
int main() {
  cin >> n >> m;
  string s;
  cin >> s;
  if (n < m) {
    cout << "YES";
    return 0;
  }
  for (int i = 0; i < n; ++i) {
    mp[s[i]]++;
    mx = max(mx, mp[s[i]]);
  }
  if (mx > m)
    cout << "NO";
  else
    cout << "YES";
}
