#include <cstdio>
#include <iostream>
#include <cmath>
using namespace std;

const double PI = 3.14159265358979323846264338327950288419716939937510582097494459230781641;

int main() {
	double a, b, c, r;
	cin >> a >> b >> c >> r;
	double p = (a + b + c) / 2;
	double S = sqrt(p * (p-a) * (p-b) * (p-c));
	double R = S / p;
	double coef = r / R;
	double saved = coef * coef * (S - PI*R*R);
	printf("%.15f\n", (S - saved) / S);
}