using System;

public class Program

{ 

        public static int  CheckValue(int[] H)

        {

            int Ch = H[1] - H[0];

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

            if(H[i+1]-H[i]!=Ch)

            return (-1);

        return (Ch);

        }

      public static void main(string[] args)

        {

        int[] H = { 1, 3, 9, 15, 17, 23, 25, 29, 33, 35 };

        int Ch =H[1] - H[0];

        if (CheckValue(H) != Ch)

                   Console.Write(-1);

            else 

                Console.Write(Ch);

         } 

       }

​