Требуется посчитать сумму целых чисел от 1 до N

var
  a:longint;
  bin,bout:text;
 
begin
assign(bin, 'INPUT.TXT');reset(bin);
assign(bout,'OUTPUT.TXT');rewrite(bout);
readln(bin, a);
if a<=0 then writeln(bout, (1+a)*(abs(a)+2) div 2)
  else writeln(bout, (1+a)*a div 2);
close(bin);close(bout);
end.

var
  a:integer; //до 10000
  bin,bout:text;
begin
assign(bin, 'INPUT.TXT');reset(bin);
assign(bout,'OUTPUT.TXT');rewrite(bout);
read(bin, a);
if a<=1 then
{a1:=a; an=1; n:=2-a; s:=(2*a+n-1)*n div 2; }
write(bout, (a+1)*(2-a) div 2)
else
{ a1:=1; n:=a; an:=a; s:=(1+a)*a div 2; }
write(bout, (1+a)*a div 2);
close(bin);close(bout);
end.

var a: integer;
    bin, bout: text;
begin
assign(bin, 'INPUT.TXT'); reset(bin);
assign(bout, 'OUTPUT.TXT'); rewrite(bout);
read(bin, a);
write(bout, (a + 1 - 2 * ord(a < 0)) * abs(a) div 2 + ord(a <= 0))
end.

var
  a:longint; 
  bin,bout:text;
begin
assign(bin, 'INPUT.TXT');reset(bin);
assign(bout,'OUTPUT.TXT');rewrite(bout);
read(bin, a);
if a<=1 then
{a1:=a; an=1; n:=2-a; s:=(2*a+n-1)*n div 2; }
write(bout, (a+1)*(2-a) div 2)
else
{ a1:=1; n:=a; an:=a; s:=(1+a)*a div 2; }
write(bout, (1+a)*a div 2);
close(bin);close(bout);
end.