using System;

​

namespace ConsoleApplication1

{

    public class Program

    {

        public static void Main()

        {

            Console.WriteLine("Please enter the height of the column.(-1 will be the last column)");

            // Inductive extension:

            int CurrentColumnHeight = 0; // Height of current column

            int LastHeigestColumn = 0; // Height of highest column entered till current column

            int Output = 0; // Sum of water level stored.

            CurrentColumnHeight = Convert.ToInt32(Console.ReadLine());

            // pre-condition: Current Column height,LastHeigestColumn and Output = 0

            // invariant: CurrentColumnHeight != -1 and Output = Sum of water level till the Previous Column + water level in current column

            while (CurrentColumnHeight != -1)

            {

                if (LastHeigestColumn < CurrentColumnHeight)

                {

                    LastHeigestColumn = CurrentColumnHeight;

                }

                else

                {

                    Output = Output + (LastHeigestColumn - CurrentColumnHeight);

                }

                CurrentColumnHeight = Convert.ToInt32(Console.ReadLine());

            }

            //post-condition: Sum of water level is stored in Output and CurrentColumn height is -1

            //Time complexity: O(n)

​

            Console.WriteLine("Output = " + Output);

            //Console.Read();

        }

    }

​

}

​