package chelper;

import java.util.HashSet;
import java.util.Set;

import io.InputReader;
import io.OutputWriter;

public class B {
  public void solve(int testNumber, InputReader in, OutputWriter out) {
    int n = in.nextInt();
    String s = in.nextString();
    Set<Integer> set = new HashSet<>();

    for (char c : s.toCharArray()) {
      set.add((int)c);
    }

    if (set.size() != n) {
      out.println("NO");
    } else {
      out.println("YES");
    }
  }
}