#include<iostream>
#include<iomanip>
using namespace std;
int main(void)
{
	long double cal, in, out;
	long double r, s, ans;
	cin >> cal >> in >> out;

	ans = cal * (in+out)*(out-in)*(out-in) /4* 3.141592653589*3.141592653589;
	cout <<fixed<<setprecision(10)<<ans << endl;

	return 0;
}