Untitled

import java.util.Scanner;

public class P1 {
	public static float Sol(int n) {
		float sum = 0;
		for (int i = n; i >= 1; i--) {
			sum += Math.sqrt(i);
		}

		return sum;
	}

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt();

		System.out.println(Sol(n));
//		Sol(n);

	}
}