#include <iostream>

using namespace std;

int main()
{
    int a,b,c,d,maxim=0,nr1;
    cin>>a>>b>>c>>d;
    nr1=a*b+c*d;
    if(nr1>maxim)  maxim=nr1;
    nr1=a*c+b*d;
    if(nr1>maxim)  maxim=nr1;
    nr1=a*d+c*b;
    if(nr1>maxim)  maxim=nr1;
    cout<<maxim;
}