using System;

​

public class Program

{

    public static void Main()

    {

        Console.WriteLine(BinarySearch(new int[]{1, 2, 3, 4, 5, 6}, 4));

    }

​

    public static int BinarySearch(int[] array, int value)

    {

        var lowerBound = 1;

        var upperBound = array.Length;

        while (lowerBound <= upperBound)

        {

            var midPoint = (lowerBound + upperBound) / 2;

            if (array[midPoint] > value)

                upperBound = midPoint - 1;

            else if (array[midPoint] < value)

                lowerBound = midPoint + 1;

            else

                return midPoint;

        }

​

        return -1;

    }

}