#include <iostream>

using namespace std;

int a, b, c, d;

int main()
{
    cin >> a >> b >> c >> d;

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

    return 0;
}