#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<random>
#include<stack>
#include<queue>
#include<cassert>
using namespace std;
int inf = 1000000000;

string sp(string a){
  for(int i=0; i<a.size(); i++){
    if( a[i] == '7' ) a[i] = '4';
    else if( a[i] == '4' ) a[i] = '7';
  }
  return a;
}

int main(void) {

  string a, b;
  cin >> a >> b;

  if( a.size() > b.size() ) cout << a << endl;
  else if( a.size() < b.size() ) cout << b << endl;
  else{
    for(int i=0; i<a.size(); i++){
      if( a[i] == b[i] ) continue;
      else if( (a[i] == '4' || a[i] == '7') && ( b[i] == '4' || b[i] == '7' ) ){
        if( a[i] == '4' ){
          cout << a << endl;
          return 0;
        }else{
          cout << b << endl;
          return 0;
        }
      }else{
        if( a[i] > b[i] ) cout << a << endl;
        else cout << b << endl;
        return 0;
      }
    }
    cout << a << endl;
  }

  return 0;
}