#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
#include <iomanip>
using namespace std;
#define Would
#define you
const int INF = 999999999;
const int MOD = 1e9 + 7;
const double pi = 3.141592653589793238;

int main() {
	double a, b, c, r, d;
	cin >> a >> b >> c;
	r = (c - b) / 2, d = (c + b) / 2;
	cout << setprecision(20) << r * r*pi * 2 * pi*d*c << endl;
}