#include <iostream>
using namespace std;

int main(){

	char alf[100];
	cin >> alf;

	char* ans = "yes";

	for(int i =0;alf[i] != '\0';i++){
		for(int j= i+1; alf[j] !='\0' ;j++){
			if(alf[i] == alf[j]){
				ans = "no" ;
			}
		}
	}

	cout <<ans;

	return 0;

}