#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)

int main()
{
	string A, B; cin >> A >> B;

	if (A.length() < B.length())
	{
		cout << B << endl;
		return 0;
	}

	if (B.length() < A.length())
	{
		cout << A << endl;
		return 0;
	}

	rep(i, 0, A.length())
	{
		if (A[i] == B[i]) continue;

		if (A[i] == '4' && B[i] == '7')
		{
			cout << A << endl;
			return 0;
		}

		if (A[i] == '7' && B[i] == '4')
		{
			cout << B << endl;
			return 0;
		}

		if (A[i] < B[i])
		{
			cout << B << endl;
			return 0;
		}

		if (A[i] > B[i])
		{
			cout << A << endl;
			return 0;
		}
	}
}