>var n,k,ans1,ans2:longint;
>
>begin
>  read(n,k);
>  if n<k then write('0') else begin
>   ans1:=(n div k)*(n + n mod k);
>   ans2:=(n-k)*4;
>  if ans1>ans2 then write(ans1) else write(ans2); end;
>end.

 My program is very short.

My method is like yours and I can't find the bug in it. What was
wrong with yours?

For 8 3 answer is 21, not 20