#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int> VI;
#define REP(i, n) for(int(i)=0;(i)<(n);++(i))
#define in(T,V) T V;cin>>V;
const int MOD = int(1e9+7);

int main(){
    in(string,A);
    in(string,B);

    string res = [&]()->string{
        if(A.length() > B.length()) return A;
        if(A.length() < B.length()) return B;
        int n = A.length();
        REP(i,n){
            if(A[i] == '4' && B[i] == '7') return A;
            if(A[i] == '7' && B[i] == '4') return B;
            if(A[i] != B[i]) return A[i]<B[i]?B:A;
        }
        return "";
    }();
    cout << res << endl;
}