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

int main() {
  string S; cin >> S;
  sort(S.begin(), S.end());
  string ans = "a";
  int s = S.find(ans);
  while(s != -1){
    ans += "a";
    s = S.find(ans);
  }
  cout << ans << endl;
}