#include <bits/stdc++.h>

using namespace std;

int main() {
  string S;
  cin >> S;
  unordered_set<char> C(S.begin(), S.end());
  if (S.length() == C.size()) {
    printf("yes\n");
  } else {
    printf("no\n");
  }
  return 0;
}
