#include <bits/stdc++.h>

using std::cin;
using std::cout;
using std::string;

int main() {
	string s;
	cin >> s;
	int n = s.size();
	sort(s.begin(), s.end());
	
	int k = 0;
	while (s[k] == s[0]) ++k;
	
	string ans = s.substr(0, k) + "a";
	cout << ans << '\n';
	
	return 0;
}