#include <bits/stdc++.h>
using namespace std;
typedef unsigned long long ul;
typedef signed long long ll;
ul over = 1000000007;

int main(void)
{
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout << fixed;
  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;
      if (a[i] == '4' && b[i] == '7') {cout << a << endl; return 0;}
      if (a[i] == '7' && b[i] == '4') {cout << b << endl; return 0;}
      cout << max(a, b) << endl; return 0;
    }
    cout << a << endl;
  }
  return 0;
}