That is, if the sum you want onto Sheet1 is in cell A10 of Sheet2.

But you can easily have this entered for you by just going to your Sheet1 (or whatever the name is) and click on the Sheet1 cell, that you want the carry over from Sheet2, and press the equals key (the =).
Then click on the tab for your Sheet2, then
click on the cell that holds that sum,
and press that Enter key.