I would greatly appreciate help with my handicap spreadsheet.

Here's the setup: players listed in column A, scores for 18 weekly games in
columns B (the first week of the season) through S (the last week of the
season). I need a formula that will calculate, for a given player, the
average of his 3 scores (if he only has three scores), or the average of his
four scores (if he only has four scores), or the average of his _most
recent_ five scores (if he has five or more scores). The formula must take
into account that a given player may not play every week. Cells
corresponding to the missed weeks would be empty.

Thanks!