#include <bits/stdc++.h>
using namespace std;
int FIRST[256];
int LAST[256];
bool USED[256];
int main() {
  ios::sync_with_stdio(0);
  string entrances;
  int guests;
  int guards;
  cin >> guests >> guards;
  cin >> entrances;
  vector<pair<int, bool>> events;
  for (int i = 0; i < guests; ++i) {
    char entry = entrances[i];
    if (!USED[entry]) {
      FIRST[entry] = i;
      LAST[entry] = i;
      USED[entry] = true;
    } else {
      LAST[entry] = i;
    }
  }
  for (char c = 'A'; c <= 'Z'; ++c) {
    if (USED[c]) {
      events.push_back(make_pair(FIRST[c], false));
      events.push_back(make_pair(LAST[c], true));
    }
  }
  sort(events.begin(), events.end());
  bool door_was_unguarded = false;
  for (auto e : events) {
    if (e.second == false) {
      guards -= 1;
    } else {
      guards += 1;
    }
    if (guards < 0) {
      door_was_unguarded = true;
    }
  }
  if (door_was_unguarded) {
    cout << "YES" << endl;
  } else {
    cout << "NO" << endl;
  }
  return 0;
}
