#include <iostream>
#include <algorithm>
#include <cmath>
#include <vector>
#include <complex>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <iomanip>
#include <assert.h>
#include <array>
#include <cstdio>
#include <cstring>
#include <random>
#include <functional>

using namespace std;

#define REP(i,a,b) for(int i=a;i<(int)b;i++)
#define rep(i,n) REP(i,0,n)

typedef long long ll;

int main() {

  string a, b; cin >> a >> b;
  string A = a, B = b;
  if(a.size() < b.size()) {
    cout << b << endl;
  }
  else if(a.size() > b.size()) {
    cout << a << endl;
  }
  else {
    rep(i, a.size()) {
      if(a[i] == '4' && b[i] == '7') {
        cout << a << endl;
        break;
      }
      else if(a[i] == '7' && b[i] == '4') {
        cout << b << endl;
        break;
      }
      else {
        if(a[i] < b[i]) {
          cout << b << endl;
          break;
        }
        else if(a[i] > b[i]) {
          cout << a << endl;
          break;
        }
      }
    }
  }

  return 0;
}