#include <iostream>
using namespace std;

int main()
{
    int a,b,c,d;
    cin>>a>>b>>c>>d;
    int max=0;
    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;
}