//https://ncode.syosetu.com/n4830bu/293/
#include <bits/stdc++.h>
using namespace std;

int main() {
    string A, B;
    cin >> A >> B;
    if (A.size() < B.size())
        cout << B << endl;
    else if (A.size() > B.size())
        cout << A << endl;
    else {
        auto comp = [](char a, char b) {
            if (a == '7' && b == '4')
                return true;
            else if (a == '4' && b == '7')
                return false;
            else
                return a < b;
        };
        for (int i = 0; i < A.size(); i++) {
            if (comp(A[i], B[i]) && !comp(B[i], A[i])) {
                cout << B << endl;
                return 0;
            }
            if (comp(B[i], A[i]) && !comp(A[i], B[i])) {
                cout << A << endl;
                return 0;
            }
        }
    }
}