//#include <atcoder/all>
#include <bits/stdc++.h>
#include <cmath>
using namespace std;
//using namespace atcoder;
using ll = long long;
#define all(A) A.begin(),A.end()
using vll = vector<ll>;
#define rep(i, n) for (long long i = 0; i < (long long)(n); i++)
using Graph = vector<vector<ll>>;



int main() {
	string S;
	cin>>S;
	string AN="a";
	rep(i,S.size()){
		bool C=true;
		rep(j,S.size()-i){
			if(AN==S.substr(j,i+1)){
				C=false;
			}
			//cout<<S.substr(j,i+1)<<endl;
		}
		if(C){
			cout<<AN<<endl;
			return 0;
		}
		AN+="a";
	}
	cout<<AN<<endl;

}