#include <bits/stdc++.h>
using namespace std;
struct less_key {
  bool operator()(pair<double, int64_t> p1, pair<double, int64_t> p2) {
    return (p1.first > p2.first) ||
           ((p1.first == p2.first) && (p1.second < p2.second));
  }
};
struct pair_hash {
  std::size_t operator()(const pair<int64_t, int64_t>& k) const {
    return static_cast<size_t>(k.first ^ k.second);
  }
};
const int64_t mod = 100000000007ll;
const int64_t inf = 10000000000000007ll;
const double eps = 0.00000001;
int64_t n, k;
string s;
int64_t amin[256];
int64_t amax[256];
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> k;
  cin >> s;
  for (int64_t i = 0; i < s.size(); i++) {
    if (!amin[s[i]]) {
      amin[s[i]] = i + 1;
    }
    amax[s[i]] = i + 1;
  }
  set<pair<int64_t, int64_t> > q;
  for (int64_t i = 'A'; i <= 'Z'; i++) {
    if (amin[i]) {
      q.insert({amin[i], 0});
      q.insert({amax[i], 1});
    }
  }
  int64_t z = 0;
  for (auto i : q) {
    if (i.second) {
      z--;
    } else {
      z++;
    }
    if (z > k) {
      cout << "YES" << endl;
      return 0;
    }
  }
  cout << "NO" << endl;
  return 0;
}
