using System;

using System.Linq;

public class Program

{

    public static void Main()

    {

            int[] numbers = new int[] {1,2,3,4,5,7};

            int result = 0;

​

            var msg = "OK";

            var strNum = string.Empty;

​

            if (numbers.Any())

            {

                result = numbers.Where(s => s < numbers.Max()).FirstOrDefault(s => !numbers.Contains(s + 1));

​

                if (result != 0)

                {

                    result++;

                    msg = "Missing Number:";

                    strNum = result.ToString();

                }

            }

​

            Console.WriteLine(string.Format("{0} {1}", msg, strNum));

    }

}