Sum of different lines

Dear all,
after some days not using excel so often my brain hurts trying to remember a solution I created long time ago (and my flash disk is not available for me this month).
I need to summarize YTD number of monthly movement on account listed on more sheets.
more in detail: I need excel make a sum (starting at cell which has same name as in A column on sheet in B column. I need it makes a summation of not all, but just YTD months (=here:offset (0, reported month).

Please give me a hint or a solution and save it in excel as well (I am not using english excel, but saving in file make all needed translation.)

Thank you very much.

Other way: See formulas I created. Make them more automatic. Thanks

Petr