#include<iostream>
#include<string>
#include<map>
using namespace std;

int main(){
	string s,out = "yes";
	map<char,bool> mp;
	cin >> s;
	for(auto i:s){
		if(mp[i]) {out = "no"; break;}
		else mp[i]=true;
	}
	cout << out << endl;
	
	return 0;
}