var k,i,j:longint;
s:int64;
begin
readln(k);i:=0;s:=0;
repeat
i:=i+1;
for j:=1 to i do begin
s:=s+i;
k:=k-1;
if k=0 then begin writeln(s);halt;end;
end;
until k=0;
writeln(s);
end.