// Arup Guha
// 1/27/2018
// Solution to 2017 MCPC Problem D: No Duplicates

import java.util.*;

public class noduplicates {

	public static void main(String[] args) {

		Scanner stdin = new Scanner(System.in);
		StringTokenizer tok = new StringTokenizer(stdin.nextLine());

		HashSet<String> set = new HashSet<String>();
		boolean ok = true;

		// Go through each token.
		while (tok.hasMoreTokens()) {
			String s = tok.nextToken();
			if (set.contains(s)) ok = false;
			set.add(s);
		}

		// Just output if there were repeats or not.
		if (ok)
			System.out.println("yes");
		else
			System.out.println("no");
	}
}