#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
  #include "settings/debug.cpp"
#else
  #define Debug(...) void(0)
#endif
#define rep(i, n) for (int i = 0; i < (n); ++i)
using ll = long long;
using ull = unsigned long long;

int main() {
  string a, b;
  cin >> a >> b;
  if (a.size() != b.size()) {
    cout << (a.size() > b.size() ? a : b) << endl;
  }
  else {
    rep(i, a.size()) {
      if (a[i] != b[i]) {
        if (a[i] == '4' && b[i] == '7') {
          cout << a << endl;
        }
        else if (a[i] == '7' && b[i] == '4') {
          cout << b << endl;
        }
        else {
          cout << (a[i] > b[i] ? a : b) << endl;
        }
        break;
      }
    }
  }
  return 0;
}