const int NOT_FOUND = -1;
static int Find(int[] arr, int num)
{
	if (arr.Length == 0)
		return NOT_FOUND;
	if (arr.Length == 1)
		return arr[0] == num ? 0 : NOT_FOUND;

	int m = arr.Length / 2;

	if (arr[m] == num)
		return m;
	int k = NOT_FOUND;
	if (arr[m] > num)
		k = Find(arr.Take(m).ToArray(), num);
	else
	{
		k = Find(arr.Skip(m).ToArray(), num);
		k = k == NOT_FOUND ? NOT_FOUND : (k + m);
	}
	return k;
}

static int NonRecursiveFind(int[] arr, int num)
{
	int start = 0;
	int end = arr.Length - 1;

	while (start != end)
	{
		int m = (start + end) / 2;

		if (arr[m] == num)
			return m;

		if (arr[m] > num)
			end = m;
		else
			start = m;
	}

	return NOT_FOUND;
}