#include <bits/stdc++.h>
#define rep(i,n) for (int i=0;i<(n);++i)
using namespace std;
int main(){
	string s;cin>>s;
	int ans=0;
	rep(i,s.length())rep(j,s.length())if(s[i]!=s[j]&&i!=j)ans++;
	cout<<(ans==(s.length())*(s.length()-1)?"yes":"no")<<endl;
}