#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int, int>;
using T = tuple<int, int, int>;
#define al(a) a.begin(), a.end()
#define ral(a) a.rbegin(), a.rend()
#define sz(a) (int)a.size()
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rrep(i, n) for (int i = 1; i <= (n); ++i)
#define drep(i, n) for (int i = (n)-1; i >= 0; --i)
#define db(a, b) cout << #a << ": " << a << " " << #b << ": " << b << endl;

int main() {
  string s, t;
  cin >> s >> t;
  if (sz(s) > sz(t))
    cout << s << endl;
  else if (sz(s) < sz(t))
    cout << t << endl;
  else {
    int n = sz(s);
    string ns = s, nt = t;
    rep(i, n) {
      if (s[i] == '4' && t[i] == '7') swap(ns[i], nt[i]);
      if (s[i] == '7' && t[i] == '4') swap(ns[i], nt[i]);
    }
    if (ns > nt)
      cout << s << endl;
    else
      cout << t << endl;
  }
  return 0;
}