#include <iostream>
using namespace std;

int main() {

	char str1[11], str2[11], stra1[11], srea2[11];
	int a1, a2, i = 0;
	int j = 0, r = 0;

	cin >> str1 >> str2;

	for (int j = 0; str1[j] == '\0'; j++) {}
	for (int r = 0; str2[r] == '\0'; r++) {}

	if (j == r) {
		for (i = 0; i == '\0'; i++) {
			if (str1[i] == 4 && str2[i] == 7) {
				cout << str1;
				break;
			}
			else if (str1[i] == 7 && str2[i] == 4)
				cout << str2;
			break;
		}
	}

	return 0;
}