package yukicoder;

import java.util.*;

public class Q454 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		double ans = 0;
		double x = sc.nextDouble();
		assert x >= 0 && x <= 1000;
		double i;
		for (i = 1; i < 100000; ++i) {
			ans += 1 / ((i + x) * (i + x));
		}
		ans += 1 / (x + i - 0.5);
		System.out.printf("%.20f\n", ans);
	}
}