It's very simple,isn't it?

var
    i,k:integer;
begin
    readln(K);
    for i:=3 to round(sqrt(k)) do if k mod i=0 then begin k:=i; break end;
    writeln(k-1);
end.

> It's very simple,isn't it?
>
> var
>     i,k:integer;
> begin
>     readln(K);
>     for i:=3 to round(sqrt(k)) do if k mod i=0 then begin k:=i; break end;
>     writeln(k-1);
> end.

if I modify it to : for i:=3 to round(sqrt(k))+100, it is AC.
if modify to round()+1 or +2 , WA
Finally, I use : for i:=3 to 10000  :(

test ur program with k = 26 and u'll know why :)

I got it ! Thank you ! ^_^
> test ur program with k = 26 and u'll know why :)