using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("entrer le rayon du cercle");

        double rayonCercle = Convert.ToDouble(Console.ReadLine());

        double surfaceCercle = 2 * Math.PI * rayonCercle; 

        double coteCaree = Math.Sqrt(surfaceCercle);

        Console.WriteLine (coteCaree);

    }//fin main

}//fin programme