#include <iostream>

using namespace std;

int main()
{int a, b, c, d, max = 0;
    cin >> a >> b >> c >> d;
    if (a * b + c * d > max)
        max = a * b + c * d;
    if (a * c + b * d > max)
        max = a * c + b * d;
    if (a * d + b * c > max)
        max = a * d + b * c;
    cout << max;

    return 0;
}