#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  string S; cin >> S;
  string alpha = "abcdefghijklm";
  map<char,int> dic;
  for(int i = 0; i < S.size(); i++) {
    dic[alpha[i]] = 0;
  }
  for(int i = 0; i < S.size(); i++) {
    dic[S[i]]++;
  }
  string ans = "";
  for (const auto& [key, value] : dic) {
   if(value == 0) {
     ans = key;
   }
  }
  if(ans == "") {
    for(int i = 0; i < alpha.size(); i++) {
      cout << alpha[i] << endl;
    }
  }else {
    cout << ans << endl;
  }
}