#include<bits/stdc++.h>
using namespace std;
using LL=long long;
using ULL=unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

const double pi = acos(-1.);

double solve(double D){
	double res=0.;
	for(double i=1.e-12; i<1;){
		double r=1.-i; if(D>r) break;
		double d=i*2.e-7;
		double c=2.*sqrt(r*r-D*D);
		res += (2.*acos(D/r)*r*r - D*c) * d;
		i+=d;
	}
	return res;
}

int main(){
	double R,H,D; cin>>R>>H>>D;
	cout<<fixed<<setprecision(10)<<(solve(D/R/2.)*H*R*R)<<endl;
	return 0;
}