program sum(input,output);
var n,result : longint;
begin
     readln(n);
     if n >= 0 then result := n * (n+1) div 2
     else result := 1 + n * (1-n) div 2;
     writeln(result);
end.

when i tried this, i got WA(in case u are wondering, 0(0+1)/2 = 0),
however when i changed to the following i got AC...

program sum(input,output);
var n,result : longint;
begin
     readln(n);
     result := 0;
     if n > 0 then result := n * (n+1) div 2
     else result := 1 + n * (1-n) div 2;
     writeln(result);
end.

not that i'm insulting the compiler but the only conclusion i can
come to is the compiler thinks that 0(0+1)/2 <> 0...