Formula: Sum two different cells over two different sheets

Hi!

Question 1: I am looking for a way to sum two different cells over two different sheets in a spreadsheet with multiple sheets. See example below:

Spreadsheet contains 10 sheets. In sheet 1, I want for a cell to be the sum of the cell C2 from sheet 2 and D12 from sheet10 only. I do not want cells in sheets between 2 and 10 to be included in the sum. What formula should I use?

Question 2: I am looking for a way to sum two cells over two different sheets (in this case, the cell number is the same in both sheets).

I found the formula = SUM(first:last!cell) but the problem is that if there are sheets between the first and the last one, it will sum every value within the selected cell over all the sheets, but I only want the specific cell in ONLY the two selected sheets to be summed, not every sheet contained within as well. What formula should I use?

Thank you!