I have many rows of data and need to sum values in the first 13
columns (col 1 - 13) and starting from the column a value first
appears. For example a value appears in Col 5 for the first time. I
want to start summing the values for the next 12 columns. And then I
also want a sum for the same row from the Col 9 (as it contains the
4th value) to Col 15.

Jan Feb Mar Apr May June July Aug Sept Oct
352 450 560 700 70 80

I want the sum of the first 13 values starting from May and Sum of
next set of values starting from Aug.