#include <bits/stdc++.h>
#define pi 3.14159265359
using namespace std;
 
int main() {
	long double a,b,c,r;
	cin >> a >> b >> c >> r;
	long double s =(a+b+c)/2;
	long double S =0;
	long double x =acos((a*a+b*b-c*c)/2/a/b)/2;
	S -=r*r/tan(x);
	S +=(pi-2*x)/2*r*r;
	x =acos((a*a+c*c-b*b)/2/a/c)/2;
	S -=r*r/tan(x);
	S +=(pi-2*x)/2*r*r;
	x =acos((b*b+c*c-a*a)/2/b/c)/2;
	S -=r*r/tan(x);
	S +=(pi-2*x)/2*r*r;
	cout << fixed << setprecision(10) << 1+S/sqrt(s*(s-a)*(s-b)*(s-c)) << "\n";
	return 0;}

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