#include<iostream>
#include<iomanip>
#include<cmath>
using namespace std;
const int dx=1<<25;
double R,H,D;
main()
{
	cin>>R>>H>>D;
	double ans=0;
	for(int cnt=0;cnt<dx;cnt++)
	{
		double r=(1-(double)cnt/dx)*R;
		if(r+r<=D)break;
		double theta=2*acos(D/2/r);
		ans+=r*r*theta-r*r*sin(theta);
	}
	cout<<fixed<<setprecision(16)<<ans*H/dx<<endl;
}