#include <algorithm>
#include <iostream>

using namespace std;

double numbers[4];

int main(int argc, const char * argv[])
{

    for (int i=0;i<4;i++){
        cin >> numbers[i];
    }
    
    double a=numbers[1]*numbers[2]+numbers[3]*numbers[0];
    double b=numbers[3]*numbers[2]+numbers[1]*numbers[0];
    double c=numbers[2]*numbers[0]+numbers[3]*numbers[1];
    
    cout << max(a,max(b,c)) << '\n';
    
    return 0;
}