octave, 68 bytes

Result

AC

Duration

1026ms

Code [DL]

[h,w]=scanf("%d%d","C");s=repmat('*',h,w);s(2:h-1,2:w-1)=' ';disp(s)

stdin

94
42

stdout

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

stderr

octave: X11 DISPLAY environment variable not set
octave: disabling GUI features