#include <iostream>
#include <set>
using namespace std ;

int main(){
	set<char> s ;
	char c ;
	while(cin >> c){
		if(s.count(c)){
			cout << "no" << endl ;
			return 0 ;
		}else{
			s.insert(c) ;
		}
	}
	cout << "yes" << endl ;
}
