#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS

#include <iostream>
#include <string>
#include <sstream>
#include <iomanip>
#include <cmath>
#include <cstring>
#include <cstdlib>
#include <algorithm>
#include <stack>
#include <queue>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <bitset>
#include <utility>
#include <numeric>
#include <iterator>

using namespace std;

using ll = long long;
using ull = unsigned long long;

const ll inf = 1ll << 60;
const ll mod = (ll)1e9 + 7;

#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()

#define print(s) cout << s;
#define println(s) cout << s << endl;
#define printd(s, f) cout << fixed << setprecision(f) << s << endl;


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

	string a0 = a;
	string b0 = b;
	for (ll i = 0; i < a.length(); i++)
	{
		if (a[i] == '4')
		{
			a[i] = '7';
		}
		else if (a[i] == '7')
		{
			a[i] = '4';
		}
	}

	for (ll i = 0; i < b.length(); i++)
	{
		if (b[i] == '4')
		{
			b[i] = '7';
		}
		else if (b[i] == '7')
		{
			b[i] = '4';
		}
	}

	println((stoll(a) > stoll(b) ? a0 : b0));
}