I know why does many people got WA!!!!!!!!!!!!
When the n is 0,the result must be 1!!!!!!!!!!!!!!
(I'm not from the English-speaking country,my English is very bad.)
If there's someting wrong,please tell me,thanks)

instead of suming u can use this formula :
1+2+3+...+n := n*(n+1) /2;
here is my prog :
ioi_khafan@hotmail.com

var
   n    : longint;
begin
  read (n);
  if  n = 0 then
      begin
           writeln ('1');
           exit;
      end
  else
  if n > 0 then
     n := ( n * (n + 1) ) div 2
  else
      begin
           n := -1 * n;
           n := ( n * (n + 1) ) div 2;
           n := n - 1;
           n := -1 * n;
      end;
  writeln (n);
end.

Var
  n                   :integer;
  sum                 :longint;
begin
  readln(n);
  if n=0 then n:=1;
  sum:=abs(n)*(abs(n)+1) div 2;
  if n<0 then sum:=1-sum;
  writeln(sum);
  readln;
end.

Thanks!
BTW，I am familiar with your english.
Beacuse I can read Chnglish at ZJU. -_-||

thanks for the help
As you see my English too not so !