import java.io.*;
import java.util.*;


public class Main_yukicoder454 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		double x = sc.nextDouble();

		double ret = 0;
		for (int i = 10_000_000; i > 0; i--) {
//			double tmp = (x + i) * (x + i);
			ret += ((double)2 * x / i + (double)x * x / i / i)  / (x + i) / (x + i);
		}

//		pr.printf("%.10f\n", ret);
		pr.printf("%.10f\n", Math.PI * Math.PI / 6 - ret);
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);

		solve();

		pr.close();
		sc.close();
	}

	private static class Printer extends PrintWriter {
		Printer(PrintStream out) {
			super(out);
		}
	}
}