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

int main(void) {
  string S;
  cin >> S;
  set<char> s;
  for (int i = 0, len = S.length(); i < len; i++) {
    s.insert(S[i]);
  }
  cout << (s.size() == S.size() ? "yes" : "no") << endl;
  return 0;
}