using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(FindElement(new int[] { }, 0));

        Console.WriteLine(FindElement(new[] { 0 }, 0));

        Console.WriteLine(FindElement(new[] { 0, 1 }, 1));

        Console.WriteLine(FindElement(new[] { 0, 1, 3, 5 }, 2));

        Console.WriteLine(FindElement(new[] { 10, 11, 13, 15, 2, 5, 7 , 8 }, 5));

        Console.WriteLine(FindElement(new[] { 10, 11, 13, 15, 2, 5, 7 , 8 }, 15));

        Console.WriteLine(FindElement(new[] { 10, 11, 13, 15, 2, 5, 7 , 8 }, 10));

        Console.WriteLine(FindElement(new[] { 10, 11, 13, 15, 2, 5, 7 , 8 }, 13));

        Console.WriteLine(FindElement(new[] { 10, 11, 13, 15, 2, 5, 7 , 8 }, 8));

    }

    static int FindElement(int[] arr, int element) {

        if(arr.Length == 0) {

            return -1;

        }

        return FindElement(arr, element, 0, arr.Length - 1);

    }

    static int FindElement(int[] arr, int element, int start, int end) {

        //Console.WriteLine("start=" + start + " end=" + end);

        var len = end - start;

        if(len <= 2) {

            if(arr[start] == element) {

                return start;

            } else if(arr[start + 1] == element) {

                return start + 1;

            }

        }

​

        var halfPos = (int) ((end + start) / 2.0);

        //Console.WriteLine("half=" + halfPos);

        if(CanBeWithinRange(arr, element, start, halfPos)) {

            return FindElement(arr, element, start, halfPos);

        } else if(CanBeWithinRange(arr, element, halfPos + 1, end)) {

            return FindElement(arr, element, halfPos + 1, end);  

        }

        return -1;

    }

    static bool CanBeWithinRange(int[] arr, int element, int start, int end) {

        return element >= arr[start] && element <= arr[end] || (arr[start] >= arr[end] && (element >= arr[start] || element <= arr[end]));

    }

}