#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

#define RREP(i,s,e) for (i = e-1; i >= s; i--)
#define rrep(i,n) RREP(i,0,n)
#define REP(i,s,e) for (i = s; i < e; i++)
#define rep(i,n) REP(i,0,n)
#define INF 1e8

typedef long long ll;

int main() {
    int i;
    string a, b, ta, tb;
    cin >> a >> b;
    if (a.size() < b.size())
        cout << b << endl;
    else if (a.size() > b.size())
        cout << a << endl;
    else {
        string ans;
        for (int i = 0; i < a.size(); i++) {
            if (a[i] == '4' && b[i] == '7') {
                ans = a;
                break;
            }
            else if (a[i] == '7' && b[i] == '4') {
                ans = b;
                break;
            }
            else if (a[i] < b[i]) {
                ans = b;
                break;
            }
            else if (a[i] > b[i]) {
                ans = a;
                break;
            }
        }
        cout << ans << endl;
    }
    return 0;
}