package yukicoder;

import java.util.*;

public class Q1436 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double res = Math.PI * Math.PI / 6;
		double ans = 0;
		double i = 1;
		double x = sc.nextDouble();
		ans = 0;
		i = 1;
		int d = (int) x;
		for (double j = 1; j <= d; ++j)
			res -= 1 / (j * j);
		for (; res - res * Math.pow(Math.E, -x / i) > 0.0000000001; ++i) {
			ans += 1 / ((i + x) * (i + x));
			res -= 1 / ((i + d) * (i + d));
		}
		ans += res;

		System.out.printf("%.20f\n", ans);
	}
}