#include <bits/stdc++.h>

using namespace std;

int main()
{
    string a, b; cin >> a >> b;
    if (a.size() > b.size()) {
        cout << a << endl;
    } else if (b.size() > a.size()) {
        cout << b << endl;
    } else {
        for (int i = 0; i < a.size(); ++i) {
            if (a[i] > b[i] || (a[i] == '4' && b[i] == '7')) {
                cout << a << endl;
                goto end;
            }
            if (b[i] > a[i] || (b[i] == '4' || a[i] == '7')) {
                cout << b << endl;
                goto end;
            }
        }
        cout << a << endl;
    }
    end:;
}