Countif / Sumif greater than zero

Hi guys I have a sheet that has two columns that are blank currently. Eventually when a number between 1 to 20 is entered into Column A I want Column B to show the result of (21 minus the value entered into Column A providing that the number in Column A is greater than zero ie 1 to 20 )
To explain Column A are points voted for and Column B is the ranking of the points.
Ie 20 points are allocated to the most favoured therefore it will rank number 1
A bit convoluted but that is the way it was set up and I am trying to convert the points to a meaningful Ranking
How do I achieve this please?
Thank you


type =IF(AND(A1>0,A1<=20),21-A1,"") into cell B1 (assuming input is into A1).

Thank you so much that did the trick. I appreciate your time and knowledge!