#include <iostream>
#include <vector>
#include <algorithm>
#include <map>
#include <queue>
#include <math.h>
using namespace std;
typedef long long int ll;

int main(){
	cin.tie(nullptr);
	ios::sync_with_stdio(false);
	double a,b,c; cin >> a >> b >> c;
	double pi=atan(1)*4;
	double ret=(b+c)/2.0*2.0*pi*((c-b)/2.0*(c-b)/2.0*pi)*a;
	printf("%.12f\n",ret);
}