#include <bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
using ll =  long long ;
using P = pair<int,int> ;
const ll INF = 1e10;
const int MOD = 1000000007;

int main(){
    string A,B;
    cin >> A >> B;
    if(A.size() > B.size()){
        cout << A << endl;
    }else if(A.size() < B.size()){
        cout << B << endl;
    }else{
        int n = A.size();
        rep(i,n){
            if(A[i]==B[i]) continue;
            if(A[i]=='4' && B[i] == '7'){
                cout << A << endl;
            }else if(A[i] =='7' && B[i] == '4'){
                cout << B << endl;
            }else if(A[i] < B[i]){
                cout << B << endl;
            }else{
                cout << A << endl;
            }
            break;
        }
    }
    return 0;
}