Sum in accordance to another cell

[gilbert]

\1

From the above diagram, can anyone help me with a formula for Accumulated cell in Last Year? Basically, this auto sum has to correspondence with the figures entered into This Year cells (ie. if 2 cells have figures, the Accumulated cells in Last Year will sum up 2 cells only and so forth)

Please help.

P/S : If you difficulties in viewing the above picture... here is a text format

Jan Feb Mac Apr May Jun Accumulated Total
Last yr 100 130 150 250 500 120 380 1250
This yr 200 150 300 650 650