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

class Ideone
{
	 public static void main(String[] args) {
        double T, l, g = 9.8075;
        Scanner scanner = new Scanner(System.in);
        l = scanner.nextDouble();
        T = 2 * Math.PI * Math.sqrt(l / g);
        System.out.println(T);
    }
}