Formula to calculate MAX from strings

I have a number of cells with week numbers listed in a column, eg:

A1 = Wk01
A2 = Wk02
A3 = Wk09
A4 = Wk32
A5 = Wk24

I would like to extract MIN and MAX week.

Could you suggest a formula?:

A10 = MAX_Formula (A1:A5) = Wk32