#include <bits/stdc++.h>
using namespace std;
typedef signed long long ll;

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

double R,H,D;

void solve() {
	int i,j,k,l,r,x,y; string s;
	double pi=atan(1)*4;
	
	cin>>R>>H>>D;
	double step=H/10000000;
	double ret=0;
	D/=2;
	for(double y=step/2;y<=H;y+=step) {
		double CR=R*y/H;
		if(CR<D) continue;
		double theta=acos(D/CR);
		
		double arc=CR*CR*theta;
		double area=CR*sin(theta)*D;
		//cout<<arc-area<<endl;
		ret+=arc-area;
	}
	
	ret*=2*step;
	_P("%.12lf\n",ret);
	
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}