#include <iostream>
using namespace std;
int main(){
    string a,b;cin>>a>>b;

    if(a.size() != b.size()){
        cout << (a.size()<b.size()?b:a) << endl;
    }else{
        for(int i = 0; a.size() > i; i++){
            if(a[i] == '4')a[i] = '7';
            else if(a[i] == '7')a[i] = '4';
            if(b[i] == '4')b[i] = '7';
            else if(b[i] == '7')b[i] = '4';
        }
        for(int i = 0; a.size() > i; i++){
            if(a[i] > b[i]){
                for(int z = 0; a.size() > z; z++){
                    if(a[i] == '4')a[i] = '7';
                    else if(a[i] == '7')a[i] = '4';
                }
                cout << a << endl;
                return 0;
            }else if(a[i] < b[i]){
                for(int z = 0; a.size() > z; z++){
                    if(b[i] == '4')b[i] = '7';
                    else if(b[i] == '7')b[i] = '4';
                }
                cout << b << endl;
                return 0;
            }
        }
    }
}