#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
double pi = 3.1415926535898;
string ys = "Yes\n";
string no = "No\n";
const double EPS = 1e-9;
const int inf = 1 << 30;
const long long M = 1e9 + 7;
const int N = 100100;
int n, m;
int f[300];
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  ;
  cin >> n >> m;
  string s;
  cin >> s;
  for (auto c : s) ++f[c];
  int ans = 0;
  for (int i = 'a'; i <= 'z'; i++) {
    if (f[i] > m) {
      ans++;
    }
  }
  if (!ans)
    cout << ys;
  else
    cout << no;
}
