#include <string>
#include <iostream>

using namespace std;

bool comp(int a, int b) {
  if ((a == '4' || a == '7') &&
      (b == '4' || b == '7')) {
    return a < b;
  }
  return a > b;
}

string comp(const string& a, const string& b) {
  if (a.size() > b.size()) return a;
  if (a.size() < b.size()) return b;
  for (size_t i = 0; i < a.size(); i++) {
    if (comp(a[i], b[i])) return a;
    if (comp(b[i], a[i])) return b;
  }
  return a;
}

int main(void) {
  string a, b;
  cin >> a >> b;
  cout << comp(a, b) << endl;
}