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

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