#include <iostream>
#include <string>

int cmp(char lhs, char rhs) {
    if (lhs == '4' && rhs == '7') return 1;
    if (lhs == '7' && rhs == '4') return -1;
    return (lhs == rhs ? 0 : lhs < rhs ? -1 : 1);
}

void solve() {
    std::string a, b;
    std::cin >> a >> b;

    if (a.length() != b.length()) {
        std::cout << (a.length() > b.length() ? a : b)
                  << std::endl;
        return;
    }

    int n = a.length();
    for (int i = 0; i < n; ++i) {
        int res = cmp(a[i], b[i]);
        if (res == 0) continue;

        std::cout << (res > 0 ? a : b) << std::endl;
        return;
    }
}

int main() {
    std::cin.tie(nullptr);
    std::ios::sync_with_stdio(false);

    solve();

    return 0;
}