golfscript, 30 bytes

Result

AC

Duration

1049ms

Code [DL]

~:w'*'*n+:a"**"' 'w((**n+@2-*a

stdin

16
32

stdout

********************************
*                              *
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*                              *
*                              *
*                              *
*                              *
*                              *
*                              *
*                              *
*                              *
*                              *
********************************

stderr