#include
int main()
{
    double b1,h1,b2,h2,a1,a2;
    scanf("%lf %lf %lf %lf",&b1,&h1,&b2,&h2);
    a1=(b1*h1)/2;
    a2=(b2*h2)/2;
    if(a1>a2)
        printf("%0.2lf",a1);
    else
        printf("%0.2lf",a2);
}