#include <iostream>
#include <string>
using namespace std;

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