#include <bits/stdc++.h>
using namespace std;
long long int i, j, k, t, q, n, m, xm, ym, ii, jj, iii, jjj, row, col, fcost,
    nn, mm;
long long int fl;
string s, s1, s2, s3;
char c[1002][1002], c1;
long long int fco, co, eo, oo, panga, cou, oj, ojv;
map<char, long long> map1, map2, map3;
long long int y[3000005], z[3000005], yf[3000000], zf[3000005], a[100002][100],
    x[1002][10002];
set<long long int> set1, set2;
vector<long long> v[4], vz, vv;
pair<int, int> pair1[102], pair2;
double floa, flo;
bool acha[100007];
vector<tuple<long long, long long, long long>> v1;
int main() {
  cin >> n >> m;
  cin >> s;
  for (i = 0; i < n; i++) {
    map1[s[i]]++;
  }
  map<char, long long>::iterator itt;
  for (itt = map1.begin(); itt != map1.end(); itt++) {
    if (itt->second > m) {
      cout << "NO";
      exit(0);
    }
  }
  cout << "YES";
  return 0;
}
