using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World");

    }

    public static Node FCA(Node node, Node a, Node b){

        // base case

        if(node == null){

            return null;

        }

        var isALeft = Covers(node.Left, a);

        var isBLeft = Covers(node.Left, b);

        if(isALeft != isBLeft){

            return node;

        }

        var nextNode = isALeft ? node.Left : node.Right;

        return FCA(nextNode, a, b);

    }

    public static bool Covers(Node node, Node targetNode){

        // base case

        if(node == null){

            return false;

        }

        // if found

        if(node == targetNode){

            return true;

        }

        return Covers(node.Left, targetNode) || Covers(node.Right, targetNode);

    }

    public class Node{

        public Node Left {get;set;}

                public Node Right {get;set;}

    }

}