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

using ll = long long;
#define rep(i, n) for(int i = 0; i < n; i++)

int main() {
	string a, b;
	cin >> a >> b;
	int sza = a.size(), szb = b.size();
	if(sza > szb) cout << a << endl;
	else if(sza < szb) cout << b << endl;
	else {
		rep(i, sza) {
			if(a[i] == b[i]) continue;
			if((a[i] == '4' && b[i] == '7') || (a[i] == '7' && b[i] == '4')) {
				if(a[i] == '4') cout << a << endl;
				else cout << b << endl;
			}
			else {
				if(a[i] > b[i]) cout << a << endl;
				if(a[i] < b[i]) cout << b << endl;
			}
			break;
		}
	}
}