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

#ifdef LOCAL
    #define eprintf(...) fprintf(stderr, __VA_ARGS__)
#else
        #define eprintf(...) 42
#endif

#define rep(i,n) for(int i=0;i<(int)(n);i++)
#define repi(i,a,b) for(int i=(int)(a);i<(int)(b);i++)
#define all(x) (x).begin(),(x).end()
#define foreach(u,v) for(auto (u) : (v))
#define pb push_back
#define mp make_pair
#define mt make_tuple

typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<ll, ll> pll;
typedef vector<ll> vl;

const int inf = 1e9;
const ll linf = 1LL<<60;
const ll mod = 1e9 + 7;
const double eps = 1e-9;

/*
*/

string solve(string a, string b)
{
  int na = a.size();
  int nb = b.size();
  if(na == nb){
    rep(i, na){
      if(a[i] == '7' and b[i] == '4') return b;
      else if(a[i] == '4' and b[i] == '7') return a;
      else if(a[i] > b[i]) return a;
      else if(a[i] < b[i]) return b;
    }
  }else if(na > nb){
    return a;
  }else{
    return b;
  }
}

int main()
{
  string a, b;
  cin >> a >> b;

  cout << solve(a, b) << endl;

  return 0;
}