Sum and lookup

[gvelix]

Hello, I need a little of help

I need to sum the value of a colum for every pack of years. The problem is that in some cases I got 28 rows for a year and in other cases more or less..So I suppose that the formula should sum when the year changes, but I dont know how to do this...

I have attached an example (the original has like 50000 rows)

Your help would be greatly appreciated!!