Draw a hollow square of # with given width
Charcoal, 6 bytes

Code:

ＮβＢββ#


Explanation:

Ｎβ        # Get input from the command line and store into β
   Ｂ      # Draw a hollow box with...
     β     #  Width β
      β    #  Height β
       #   #  Filled with the character '#'
           # Implicitly output the box


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