/* package whatever; // don't place package name! */

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

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);
		double T, l, g = 9.81, Pi = 3.14, s;
		l = sc.nextDouble();
		s = Math.sqrt(l/g);
		T = 2 * Pi * s;
		System.out.println(T);
	}
}