#include <iostream>
using namespace std;

string A, B;

void Solve() {
    if (A.size() != B.size()) {
        cout << (A.size() > B.size() ? A : B) << endl;
    } else {
        string answer;
        for (int i = 0; i < A.size(); i++) {
            if (A[i] == B[i]) continue;
            if (A[i] == '4' && B[i] == '7') { answer = A; break; }
            else if (A[i] == '7' && B[i] == '4') { answer = B; break; }
            else {
                if (A[i] > B[i]) { answer = A; break; }
                else { answer = B; break; }
            }
        }
        cout << answer << endl;
    }
}

int main() {
    cin >> A >> B;

    Solve();
    return 0;
}