#include<iostream>
using namespace std;
int a,b,c,d;

int main(void) {
    cin>>a>>b>>c>>d;
    int r1=a*b+c*d, r2=a*c+b*d, r3=a*d+b*c;
    cout<<max(max(r1,r2),r3);
    return(0);
}