#include <bits/stdc++.h>
#define rep(i, l, r) for (int i = (l); i < (r); i++)
using namespace std;

typedef long long ll;

int main() {
    string S;
    cin >> S;
    int x = 0, y = 0;
    rep(i, 0, S.size()) {
        if (S[i] == 'a') {
            x++;
        } else {
            y = max(x, y);
            x = 0;
        }
    }
    y = max(x, y);
    rep(i, 0, y + 1) cout << "a";
    cout << endl;
}