#include <bits/stdc++.h>
using namespace std;
const long long INF = 1e14;
const long long N = 2e5 + 1;
const long long mod = 1e9 + 7;
const long double eps = 1E-7;
long long n, m, cnt, a, b, c, used[N];
string s, t;
vector<long long> v;
int main() {
  string dp[2] = {"NO", "YES"};
  ios_base::sync_with_stdio(0);
  ;
  cin >> n >> m >> s;
  for (int i = 0; i < n; i++) used[int(s[i])]++;
  sort(s.begin(), s.end());
  for (int i = 0; i < n; i++)
    if (s[i] != s[i + 1]) t += s[i];
  for (int i = 0; i < t.size(); i++) v.push_back(used[int(t[i])]);
  for (int i = 0; i < v.size(); i++)
    if (v[i] - m > 0ll) return cout << dp[false], 0;
  cout << dp[true];
}
