#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef string str;
int main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0); cout.tie(0);
    str s1, s2;
    cin >> s1 >> s2;
    if (s1.size() != s2.size())
    {
        if (s1.size() >= s2.size()) cout << s1;
        else cout << s2;
    }
    else
    {
        bool equall = true;
        for (ll i = 0; i < s1.size(); i++)
        {
            if (s1[i] != s2[i])
            {
                equall = false;
                if (s1[i] != '4' && s1[i] != '7')
                {
                    if (s1[i] > s2[i]) cout << s1;
                    else cout << s2;
                    break;
                }
                else if (s1[i] == '4')
                {
                    if (s2[i] != '7')
                    {
                        if (s1[i] > s2[i]) cout << s1;
                        else cout << s2;
                        break;
                    }
                    else
                    {
                        cout << s1;
                        break;
                    }
                }
                else if (s1[i] == '7')
                {
                    if (s2[i] != '4')
                    {
                        if (s1[i] > s2[i]) cout << s1;
                        else cout << s2;
                        break;
                    }
                    else
                    {
                        cout << s2;
                        break;
                    }
                }
            }
        }
        if (equall) cout << s1;
    }
    return 0;
}