#include <iostream>
#include <iomanip>
#include <cmath>
double area(double a, double b, double c){ // Heron's formula for triangle area.
	double s = (a+b+c)/2;
	return sqrt(s * (s-a) * (s-b) * (s-c));
}
int main(){
	double a,b,c,d;
	std::cin >> a >> b >> c >> d; //  vvvvvvvvvvv -- cos(alpha) -- vvvvvvvvvvvv
	double f = sqrt(a*a + b*b - 2*a*b*(a*a + b*b - c*c - d*d)/(2 * (a*b + c*d)));
	std::cout << std::setprecision(16) << area(a,b,f) + area(c,d,f) << std::endl;
}