kotlin, 169 bytes

Result

AC

Duration

3790ms

Code [DL]

fun main(a:Array<String>){
var h=readLine()!!.toInt();
var w=readLine()!!.toInt()-2;
var a="*".repeat(w);
print("*$a*\n"+("*"+" ".repeat(w)+"*\n").repeat(h-2)+"*$a*");
}

stdin

19
32

stdout

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

stderr

/tmp/code.kt:4:5: warning: name shadowed: a
var a="*".repeat(w);
    ^