Hoping someone can help.

I have a sheet that has 12 months of data going across 12 columns. Each
column has a heading for each month i.e 1, 2 ,3 ... There are multiple rows
of data. I need a formula that will sum the columns of data based on the
month that is entered in cell A1. For example, if I enter 5 in cell A1, I
want the YTD formula to add up only the first 5 months of data.

Thanks in advance for any help.