#include <iostream>
#include <string>
using namespace std;

int main() {
	string s;
	cin >> s;
	
	string a;
	for (int i = 1; ; i++) {
		a += "a";
		if (s.find(a) == std::string::npos) {
			cout << a << endl;
			break;
		}
	}
	return 0;
}