Total sum of the cells from multiple rows and 2 sheets

Hello.
I'm working with quite a large material and would appreciate your help a lot.
The information I need is in two sheets (which is not a real problem) but I've tried a lot of different combinations of functions and still can't get it done correctly. In first column there is an ID-number which is same for each person in both sheets. Each ID may have varying number of transactions (each in its own row). I would like to calculate the total values (not the number of the cells meeting the criteria) of each ID from both worksheets into a single cell. Is there any way getting it done?