#include <bits/stdc++.h>
using namespace std;
using ll=long long;
ll mod=1000000007;
ll INF=1LL<<60;

int main() {
	string A,B;
	cin >> A >> B;
	if(A.size()>B.size()){
		cout << A << endl;
		return 0;
	}
	else if(A.size()<B.size()){
		cout << B << endl;
		return 0;
	}
	for(int i=0;i<A.size();i++){
		if(A[i]=='4' && B[i]=='7'){
			cout << A << endl;
			return 0;
		}
		if(A[i]=='7' && B[i]=='4'){
			cout << B << endl;
			return 0;
		}
		if(A[i]>B[i]){
			cout << A << endl;
			return 0;
		}
		if(A[i]<B[i]){
			cout << B << endl;
			return 0;
		}
	}
}