#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;

int main(){
  string a, b;
  cin >> a >> b;
  if (sz(a) != sz(b)) {
    cout << (sz(a) > sz(b) ? a : b) << endl;
  } else {
    rep(i,sz(a)) {
      int c1 = a[i] - '0', c2 = b[i] - '0';
      if (c1 == c2) continue;
      if (c1 == 4 && c2 == 7) {
        cout << a << endl;
        return 0;
      } else if (c1 == 7 && c2 == 4) {
        cout << b << endl;
        return 0;
      } else {
        cout << (c1 > c2 ? a : b) << endl;
        return 0;
      }
    }
  }
  return 0;
}