#include<iostream>
#include<string>

int main() {
	std::string a, b;
	std::cin >> a >> b;

	std::string c = a, d = b;
	for (int i = 0; i < a.length(); i++) {
		if (a[i] == '4' && b[i] == '7') {
			c[i] = '7';
			d[i] = '4';
		}
		else if (a[i] == '7'&& b[i] == '4') {
			c[i] = '4';
			d[i] = '7';
		}
	}

	long long e = std::stoi(c);
	long long f = std::stoi(d);

	if (e > f) {
		std::cout << a << std::endl;
		return 0;
	} else {
		std::cout << b << std::endl;
		return 0;
	}
}