#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define rep(i, n) for(ll i = 0, i##_len = (n); i < i##_len; i++)
#define reps(i, s, n) for(ll i = (s), i##_len = (n); i < i##_len; i++)
#define rrep(i, n) for(ll i = (n) - 1; i >= 0; i--)
#define rreps(i, e, n) for(ll i = (n) - 1; i >= (e); i--)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())
#define len(x) ((ll)(x).length())
#define endl "\n"

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    // ifstream in("input.txt");
    // cin.rdbuf(in.rdbuf());
    string a, b;
    cin >> a >> b;
    ll la = len(a), lb = len(b);
    if (la != lb) {
        cout << ((la > lb) ? a : b) << endl;
        return 0;
    }
    rep(i, la) {
        if (a[i] == b[i]) continue;
        if ((a[i] == '4') && (b[i] == '7')) {
            cout << a << endl;
            return 0;
        }
        else if ((a[i] == '7') && (b[i] == '4')) {
            cout << b << endl;
            return 0;
        }
        else {
            cout << ((a[i] > b[i]) ? a : b) << endl;
            return 0;
        }
    }
    return 0;
}