Summing a non continuous range

Hi

From a previous post I was provided with a formula to provide a year to date
total from a table based on the month I entered in A1.

=SUM(OFFSET($A3,,,1,MATCH($A$1,$A$2:$L$2,0)))

Jan Feb Mar Apr May ................................ Dec
1 1 1 1 1 1 .................................. 1
2
3

This has worked fine but I have now been presented with another table which
has a sub total column which I need to take out when the MATCH includes
certain months.

Jan Feb Mar Jan-Mar Apr May June Apr-June July ...........
Total Total
1 1 1 3 1 1 1 3
1 ............

If I use the original formula when I enter Apr the sum includes Jan, Feb,
Mar and Jan-Mar. I need a formula which removes or ignores Jan-Mar for Apr,
May and June and when I get to July will ignore/remove Jan-Mar ie July
entered in A1 should give me 7 not 13 which the original formula does.

I hope I have made this clear enough and appreciate any assistance.