//2 works

using System;

​

int[] arr = new int[6] { 6, 8, 12, 14, 16,18 };

​

int MissingNum(int[] arr)

{

    int difrents;

    int check1 = arr[1] - arr[0];

    int check2 = arr[2] - arr[1];

    int check3 = arr[3] - arr[2];

​

    if (check1 == check2 || check1 == check3)

        difrents = check1;

    else

        difrents = check2; 

​

    for (int i = 0; i < arr.Length - 1; i++)

    {

        if (arr[i + 1] - arr[i] != difrents)

        {

            return arr[i] + difrents;

        }

    }

​

    return 0; 

}

Console.Write(MissingNum( arr))