I have a worksheet with 13 columns (Column A to M) for all the months of
year plus the accumulated or Year to date total column. A formula should add
the accumulated for each month of the year in the 13th column. Example if I
input "3" in cell A1 it should automatically sum at cell M2 all three cells
for months of Jan, Feb and March or (A2,B2,C2) and if I entred "5" for months
of Jan to May and etc.

As example below if I entered "3" in cell A1, the accumulated column should
show "35". Calculated 10+15+10 for jan,feb and march figures.

A B C D .... L M
"3"
Jan Feb Mar Apr... Dec Total Accumulated/YTD
10 15 10 20 ... 12 = 35

Appreciate for your help!