#include "bits/stdc++.h"

#define REP(i, n) for(decltype(n) i = 0; i < (n); i++)
#define REP2(i, x, n) for(decltype(x) i = (x); i < (n); i++)
#define RREP(i, n) for (decltype(n) i = (n) - 1;i >= 0; i--)

#define ALL(a) (a).begin(),(a).end()
#define SORT(c) sort((c).begin(),(c).end())
#define DESCSORT(c) sort(c.begin(), c.end(), greater<int>())

#define LL long long int
#define LD long double

#define PI 3.14159265358979

#define _CRT_SECURE_NO_WARNINGS

using namespace std;

//================================================

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);

	string A, B;
	cin >> A >> B;

	if (A.size() < B.size()) {
		cout << B << "\n";
		return 0;
	} else if (A.size() > B.size()) {
		cout << A << "\n";
		return 0;
	}

	string svrA = A, svrB = B;
	REP(i, A.size()) {
		REP(j, B.size()) {

			if (i != j) continue;
			if (A[i] == '4' && B[j] == '7' || A[i] == '7' && B[j] == '4') {
				swap(svrA[i], svrB[j]);
			}
		}
	}

	int a = stoi(svrA);
	int b = stoi(svrB);

	if (a < b) {
		cout << B << "\n";
		return 0;
	} else {
		cout << A << "\n";
		return 0;
	}
	return 0;
}