#include<bits/stdc++.h>
using namespace std;
int main()
{

string s;
int a,b,i,j,l;
set<char>q;
cin>>s;
for(i=0;i<s.size();i++)
{
  q.insert(s[i]);
}
if(s.size()==q.size())
cout<<"yes\n";
else cout<<"no\n";
}