#include <iostream>
using namespace std;
int a, b, c, d, sol;

int main()
{
	cin >> a >> b >> c >> d;
	sol = a * b + c * d;
	sol = max(sol, a * c + b * d);
	sol = max(sol, a * d + b * c);
	cout << sol << "\n";
	return 0;
}