multidimensional array select corner numbers C#

Hi All,

I have a question regarding  multidimensional array.
  int[,] arr = {    {1,2,3,4},  {5,6,7,8}, {9,10,11,12},{13,14,15,16}    };

need to get the yellow color numbers and blue color numbers as separated outputs using nested for loop in C#.
Thank you in advanced.