#include <iostream>
using namespace std;

int main() {
    //long long a, b;
    string a, b;
    cin >> a >> b;
    string ans;
    if(a.size() > b.size())
        ans = a;
    else if(a.size() < b.size())
        ans = b;
    else{
        for(int i=0;i<a.size();i++) {
            if(a[i] == b[i]) continue;

            if(a[i] == '4' && b[i] == '7')
                ans = a;
            else if(a[i] == '7' && b[i] == '4')
                ans = b;
            else if(a[i] > b[i])
                ans = a;
            else if(a[i] < b[i])
                ans = b;
            break;
        }
    }

    cout << ans << endl;
}