#include <iostream>
using namespace std;

int main() {

  string S;
  cin>>S;
  
  for(int i=0;i<S.size();i++)
    for(int j=i+1;j<S.size();j++)
      if(S.at(i)!=S.at(j)) continue;
      else {cout <<"no"<< endl;return 0;}
  cout <<"yes"<< endl;
}