#include <iostream>

int cmp(long long a, long long b)
{
    int n = 0;
    long long aa=0, bb=0;
    while (a != 0 || b != 0) {
        long long an = a % 10;
        long long bn = b % 10;
        if (an == 7 && bn == 4) {
            an = 4;
            bn = 7;
        }
        else if (an == 4 && bn == 7) {
            an = 7;
            bn = 4;
        }
        if (a != 0) {
            for (int m = 0; m < n; m++) {
                an *= 10;
            }
            aa += an;
            a /= 10;
        }
        if (b != 0) {
            for (int m = 0; m < n; m++) {
                bn *= 10;
            }
            bb += bn;
            b /= 10;
        }
        n++;
    }
    return aa > bb;
}

int main()
{
    long long a, b;
    std::cin >> a;
    std::cin >> b;
    if (cmp(a, b)) std::cout << a << std::endl;
    else std::cout << b << std::endl;
    return 0;
}