#include <iostream>
#include <iomanip>
#include <fstream>
#include <cmath>
#define PI 3.141592653589793238462643383279

using namespace std;


double a,b,c,r;
double s,sm;

inline double heron(double a, double b, double c)
{
    double p=(a+b+c)/2;
    return sqrt(p*(p-a)*(p-b)*(p-c));
}

inline double p(double a)
{
    return a*a;
}

int main() {
    cin>>a>>b>>c>>r;
    
    sm=heron(a,b,c);
    s+=PI*r*r;
    double d1 = r/tan(acos((a*a+b*b-c*c)/(2*a*b))/2);
    double d2 = r/tan(acos((a*a+c*c-b*b)/(2*a*c))/2);
    double d3 = r/tan(acos((c*c+b*b-a*a)/(2*c*b))/2);
    a=a-d1-d2;
    b=b-d1-d3;
    c=c-d2-d3;
    s+=r*(a+b+c);
    s+=heron(a,b,c);
    cout<<setprecision(40);
    cout<<s/sm;
    return 0;
}