#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
int main() {
	string s;
	cin >> s;
	int ans = 0, co = 0;
	for (int i = 0; i < s.size(); i++) {
		if (s[i] == 'a') {
			co++;
		}
		else {
			co = 0;
		}
		ans = max(ans, co);
	}
	for (int i = 0; i <= ans; i++) {
		cout << 'a';
	}
	cout << endl;
}