public static int hourglassSum(List<List<int>> arr)
    {
       int maxSum = -100;
       for(int i=0; i< arr.Count-2; i++)
       {          
          for(int j=0; j<arr.Count-2; j++)
          {
              int sum=arr[i][j] + arr[i][j+1] + arr[i][j+2] 
                              + arr[i+1][j+1] 
                    + arr[i+2][j] + arr[i+2][j+1] + arr[i+2][j+2];
                     
             if(maxSum < sum) 
             {
                maxSum = sum;
             }      
          }     
       }
       return maxSum;
    }