#include <iostream>
#include <string>

using namespace std;


int main(void) {
	string a, b;
	string ao, bo;
	cin >> a >> b;
	ao = a;
	bo = b;

	if (a.size() < b.size()) {
		cout << b << endl;;
		return 0;
	}
	else if (a.size() > b.size()) {
		cout << a << endl;;
		return 0;
	}

	for (int i = 0; i < a.size(); ++i) {
		if (a[i] == '4' && b[i] == '7')
			swap(a[i], b[i]);
		else if (a[i] == '7' && b[i] == '4')
			swap(a[i], b[i]);
	}

	int aa = atoi(a.c_str());
	int bb = atoi(b.c_str());

	if (aa < bb) {
		cout << bo << endl;
	}
	else {
		cout << ao << endl;
	}

	return 0;
}