#include <cstring>
#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <queue>
#include <algorithm>
#include <stack>
#include <cassert>

int main() {

  std::string A, B;
  std::cin >> A >> B;

  // step 1
  if( A.size() != B.size() ) {
    if( A.size() < B.size() ) {
      std::cout << B << std::endl;
    }
    if( A.size() > B.size() ) {
      std::cout << A << std::endl;
    }
    return 0;
  }

  // step 2
  assert( A.size() == B.size() );
  for(int i = 0; i < (int)A.size(); ++i) {
    if( A[i] != B[i] ) {
      if( A[i] == '4' and B[i] == '7' ) {
        std::cout << A << std::endl;
      }
      else if( A[i] == '7' and B[i] == '4' ) {
        std::cout << B << std::endl;
      }
      else {
        if( A[i] < B[i] ) {
          std::cout << B << std::endl;
        }
        else if( A[i] > B[i] ) {
          std::cout << A << std::endl;
        }
      }
      return 0;
    }
  }
  
  return 0;
}