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

#define REP(i, n) for(int i=0; i<(n); i++)

int N;
string S, T;
signed main()
{
    cin >> S >> T;
    if (S.size() > T.size()) {
        cout << S << endl;
        return 0;
    }
    if (S.size() < T.size()) {
        cout << T << endl;
        return 0;
    }
    string ans = S;
    REP(i,S.size()) {
        if (S[i] == '4' && T[i] == '7') {
            ans = S;
            break;
        }
        if (S[i] == '7' && T[i] == '4') {
            ans = T;
            break;
        }
        if (S[i] != T[i]) {
            ans = (S[i] > T[i]) ? S : T;
            break;
        }
    }
    cout << ans << endl;
    return 0;
}