#include <bits/stdc++.h>
using namespace std;
 
int main() {
string s;
  cin>>s;
  int n =s.size();
  string ans="yes";
  for (int i = 0; i < n; i++) {
 for (int j = i+1; j < n; j++) {
 if(s[i]==s[j]){
   ans="no";
 }
}
}
cout<<ans<<endl;
 
 
}