#include <iostream>

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

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;
}