#include <iostream>
#include <algorithm>
using namespace std;


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