#include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <utility>
#include <cmath>

using namespace std;

int main() {
	int c, in, out;

	cin >> c >> in >> out;
	double r = 1.0 * (out - in) / 2;
	double w = 1.0 * (out + in) / 2;

	double v = M_PI * r * r * 2 * M_PI * w * c;
	printf("%.5f\n", v);
	return 0;
}