#include <bits/stdc++.h>

using namespace std;
typedef long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define MOD 1000000007
#define INF (1LL<<25)     //33554432
#define PI 3.14159265359
#define EPS 1e-12
//#define int ll

signed main(void)
{
  string a, b;
  cin >> a >> b;

  if(a.size() > b.size()) {
    cout << a << endl;
    return 0;
  } else if(a.size() < b.size()) {
    cout << b << endl;
    return 0;
  }

  REP(i, a.size()) {
    //cout << a[i] << " " << b[i] << endl;
    if(a[i] == b[i]) continue;
    if(a[i] == '4' && b[i] == '7') {
      cout << a << endl;
      //cout << "a" << endl;
      return 0;
    } else if(a[i] == '7' && b[i] == '4') {
      cout << b << endl;
      //cout << "b" << endl;
      return 0;
    } else if(a[i] > b[i]) {
      cout << a << endl;
      //cout << "c" << endl;
      return 0;
    } else if(a[i] < b[i]) {
      cout << b << endl;
      //cout << "d" << endl;
      return 0;
    }
  }

  return 0;
}