Implement the following OCaml function as a dafny function:

$/*$ let mystery$\_$function x y z $=$

let a $=$ x $+$ y in

let b $=$ y $+$ z in

let c $=$ x $+$ z in $($a$+$b$+$c$)/2$

$*/$

function mystery$\_$function$($x:int,y:int,z:int$)$:int $\{$

$/*$ Replace with your definition $*/$

$42$

$\}$