Fill column A and B with row numbers.

In C1 cell, enter =, then click on A1 will add A1 after = sign. Then enter + and click B1. The formula will be generated as =A1+B1. Press Enter to accept it.

In C2 cell, enter =sum(, then drag A1 to A10. Press enter. The formula will be generated as =sum(A1:A10). Press Enter to accept it. Note: In previous version, need to enter =total(A1:A10)

The selected cell/range shows with a light green background.

Referencing to different sheet in the same book is also supported. Different book is not supports yet.