> program ex;
> var
>   now,i,l,k:longint;
> begin
>   readln(k);
>   now:=k;
>   for i:=3 to trunc(sqrt(100000000)) do
>    if k mod i=0 then begin now:=i; break; end;
>   writeln(now-1);
> end.
>

program ex;
var
now,i,l,k:longint;
begin
readln(k);
now:=k;
for i:=3 to 10000 do {trunc(sqrt(100000000))=10000}
if k mod i=0 then begin now:=i; break; end;
writeln(now-1);
end.

I have tried.
Itself so thought.
Wrong answer.

I did so at the first time
but got WA
Try this test
1999966
the correct answer is 999982
but the result of your program is 1999965
...

In this problem, trunc(sqrt(100000000)) isn't a right limit.
We should use (k div 2)+1 for the high limit,isn't it?

var
  n,l,i:longint;
begin
  readln(n);
  l:=n-1;
  if ((n mod 2)=0) and (n div 2>2) then l:=n div 2-1;
  for i:=2 to trunc(sqrt(n)) do
    if n mod (i+1)=0 then begin
      l:=i;
      break
    end;
  writeln(l)
end.