#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;

int main()
{
	string A, B,ans;
	cin >> A >> B;

	if (A.size() > B.size()) {
		ans = A;
	}
	else {
		ans = B;
	}

	if (A.size() == B.size()) {
		for (int i = 0; i < A.size(); i++) {
			if (A[i] != B[i]) {
				if ((A[i] == '4' && B[i] == '7') || (A[i] == '7' && B[i] == '4')) {
					if (A[i] == '7' && B[i] == '4') {
						ans = B;
						break;
					}
					else {
						ans = A;
						break;
					}

				}
				else {
					if ((A[i] - '0') > (B[i] - '0')) {
						ans = A;
						break;
					}
					else {
						ans = B;
						break;
					}
				}
			}
		}
	}
	cout << ans << endl;
	return 0;
}