Google Sheets – How to increase the formula by increment of 10

I have a spreadsheet with a value per year. I would like to take the average of decades (with, obviously, 10 cells per decade), so I establish a formula like that of = MEDIUM (A (1 + 10) :( A (10 + 10), with the hope of being able to drag it so that each cell is filled with A11: A20, A21: 30, etc. How I do this? I'm not particularly smart with the formulas in Google Sheets.