#include <bits/stdc++.h>
using namespace std;

int main(){
	
	string s;
	cin >> s;
	set<char> setic(s.begin(),s.end());
	vector<char> v(s.begin(), s.end());
	(int) setic.size() == (int) v.size() ? cout << "yes" : cout << "no";
	
}

