MaxSumOfElementsK

using System;
using System.Diagnostics;


class MaxSumOfElementsK
{
    static void Main()
    {
        Console.Write("Enter n=");
        int n = int.Parse(Console.ReadLine());
        Console.Write("Enter k=");
        int k = int.Parse(Console.ReadLine());
        int firstSum = 0;
        int biggestSum = 0;
        int lastSum = 0;

        int[] arr = new int [n];

        for (int i = 0; i < n; i++)
        {
            Console.Write("Enter arr index [{0}]=",i);
            arr[i] = int.Parse(Console.ReadLine()); //Read number from Console
        }
        Stopwatch chronometer = new Stopwatch();
         chronometer.Start();
        for (int i = 0; i < arr.Length; i++)// changing index of arr
        {
            if (i+k-1 < arr.Length)           // Stop before going out of arr boundry
            {

                if (i==0)                   //Calculating the first sequence k
                {
                    for (int j = 0; j < k; j++)
                    {
                        firstSum = firstSum + arr[i + j];
                        lastSum = firstSum;
                    }

                }
                else                        //Calculating the others sequence
                {
                    lastSum = (lastSum - arr[i - 1]) + arr[i + k - 1];
                }
            }
            if (lastSum > biggestSum)      //Putting the biggest sum in variable
            {
                biggestSum = lastSum;
            }
        }
        Console.WriteLine("The biggest sum is {0}",biggestSum);
        chronometer.Stop();
        Console.WriteLine(chronometer.Elapsed);
    }
}