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

int main() {
  string S, T, ans;
  cin >> S >> T;
  if (S.size() != T.size()) return cout << ((S.size() > T.size()) ? S : T) << "\n", 0;
  for (int i = 0; i < S.size(); i++) {
    if (S.at(i) == T.at(i)) continue;
    if (S.at(i) == '4' && T.at(i) == '7') return cout << S << "\n", 0;
    if (S.at(i) == '7' && T.at(i) == '4') return cout << T << "\n", 0;
    if (S.at(i) > T.at(i)) return cout << S << "\n", 0;
    if (S.at(i) < T.at(i)) return cout << T << "\n", 0;
  }
}