#include<iostream>
#include<vector>
#include<algorithm>
#include<cmath>
#include<string>
#include<iomanip>
#include<numeric>
#include<queue>
#include<deque>
#include<stack>
#include<set>
#include<map>
#include<random>
using namespace std;
typedef long long ll;
const int mod=1e9+7;
int main(){
	string s;
	cin>>s;
	int ans=0,tmp=0;
	for(auto x:s){
		if(x=='a')
			tmp++;
		else
			tmp=0;
		ans=max(ans,tmp);
	}
	cout<<string(ans+1,'a')<<endl;
}