#include <bits/stdc++.h>
using namespace std;
int main(){
	string a,b;	cin>>a>>b;
	if(a.size()!=b.size()){
		cout<<((a.size()<b.size())?b:a)<<endl;
		return 0;
	}
	for(int i=0;i<(int)a.size();i++){
		if(a[i]=='7'&&b[i]=='4'){
			cout<<b<<endl;
			return 0;
		}
		else if(a[i]=='4'&&b[i]=='7'){
			cout<<a<<endl;
			return 0;
		}
		else if(a[i]!=b[i]){
			cout<<((a[i]>b[i])?a:b)<<endl;
			return 0;
		}
	}
}