I am trying to sum a column with the sum function say : sum(a1:a12)
however I don't always want it to add row 1 to row 12

depending on an inputted number I want to add a3 to a12, or a5 to a12, or
somthing like that
is there a way I can make the sum function do that or is there any other way
to sum up a column from row x to row 12 or something like that