#include <bits/stdc++.h>
#define GET_MACRO(a, b, c, NAME, ...) NAME
#define rep(...) GET_MACRO(__VA_ARGS__, rep3, rep2)(__VA_ARGS__)
#define rep2(i, a) rep3 (i, 0, a)
#define rep3(i, a, b) for (int i = (a); i < (b); i++)
#define repr(...) GET_MACRO(__VA_ARGS__, repr3, repr2)(__VA_ARGS__)
#define repr2(i, a) repr3 (i, 0, a)
#define repr3(i, a, b) for (int i = (b) - 1; i >= (a); i--)
#define chmin(a, b) ((b) < a && (a = (b), true))
#define chmax(a, b) (a < (b) && (a = (b), true))
using namespace std;
typedef long long ll;

int main() {
	string A, B;
	cin >> A >> B;
	string a = A;
	string b = B;
	if (A.length() > B.length()) {
		cout << A << endl;
		return 0;
	}
	if (A.length() < B.length()) {
		cout << B << endl;
		return 0;
	}
	int n = A.length();
	rep (i, n) {
		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]);
		}
		if (A[i] < B[i]) {
			cout << b << endl;
			return 0;
		} else if (A[i] > B[i]) {
			cout << a << endl;
			return 0;
		}
	}
	return 0;
}