// Pascal
program zab;
var a,n : integer;
    s:longint;
begin
     assign(input, 'input.txt'); 
     reset(input);
     assign(output, 'output.txt'); 
     rewrite(output);
     read(a);
     if (a=1) or (a=0)
       then
         s:=1
       else
       begin
     if (a>=-10000) and (a<=10000)
       then
         begin
           if a<0
             then
               n:=abs(a)+2
             else
               n:=a;
           s:=trunc((1+a)*n/2);
         end
       else
         s:=0;
       end;  
     write(s)
end.