#include <bits/stdc++.h>
using namespace std;

string a,b;

string jud()
{
    for(int i = 0; i < a.length(); i++){
        if(a[i] != b[i]){
            if(a[i] == '4' && b[i] == '7') return a;
            if(a[i] == '7' && b[i] == '4') return b;
            if(a[i] > b[i]) return a;
            if(a[i] < b[i]) return b;
        }
    }
}

int main()
{
    cin >> a >> b;

    if(a.length() > b.length()) cout << a << endl;
    else if(a.length() < b.length()) cout << b << endl;
    else cout << jud() << endl;

    return 0;
}