#include <iostream> // [ABCD] with sides a,b,c,d
#include <iomanip>  // angle DAB=alpha, angle BCD=180-alpha
#include <cmath>    // Solve cos(alpha) from:
int main(){         // a^2+b^2-2ab*cos(alpha) = f = c^2+d^2-2cd*cos(180-alpha)
	long double a,b,c,d; // Use [ABD] = a*b*sin(alpha) = a*b*sqrt(1-cos(alpha)^2)/2
	std::cin >> a >> b >> c >> d;
	std::cout << std::setprecision(16) << (a*b+c*d)*
		sqrt(1 - pow((a*a + b*b - c*c - d*d)/(2 * (a*b + c*d)),2))/2 << '\n';
} //    ^sin(alpha)  ^^^^^^^^^^ -- cos(alpha) -- ^^^^^^^^^^^^^