#include<bits/stdc++.h>
using namespace std;
map<char,int> mapa;

int main(){
	string str;
	cin >> str;
	bool f = 0;
	for(auto it:str){
		mapa[it]++;
		if(mapa[it]>=2)
			f = 1;
	}
	if(f == 1)
		cout<<"no";
	else
		cout<<"yes";
	

}