import java.util.Scanner;
import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        while(sc.hasNext()){
          String s = sc.next();
          char[] str = s.toCharArray();
          TreeSet set = new TreeSet();
            for (int i = 0; i < s.length(); i++) {
                set.add(str[i]);
            }
           if(set.size() == s.length()) System.out.println("yes");
           else System.out.println("no");
        }
    }
}