#include<bits/stdc++.h>
using namespace std;

int main(){

  string a, b;

  cin >> a >> b;

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

}