#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <sstream>

#define FORR(i,b,e) for(int i=(b);i<(int)(e);++i)
#define FOR(i,e) FORR(i,0,e)
#define ALL(c) begin(c), end(c)
#define dump(var) cerr << #var ": " << var << "\n"
#define dumpc(con) for(auto& e: con) cerr << e << " "; cerr<<"\n"

using namespace std;


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    string A, B;
    cin >> A >> B;

    if (A.size() != B.size()) {
        cout << (A.size()>B.size()? A: B) << endl;
    }
    else {
        string *ans;
        FOR(i, A.size()) {
            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 = A[i]>B[i]? &A: &B;
                break;
            }
        }
        cout << *ans << endl;
    }
    
    return 0;
}