Here's the source can you help me!

var i,sum1,n:longint;
begin
   readln(n);
   sum1:=0;
   if n=1 then sum1:=2
   else
     if n<=0 then
       for i:=1 downto n do  sum1:=sum1+i;
     else
       for i:=1 to n do  sum1:=sum1+i;
   writeln(sum1);
end.