program C;

{$APPTYPE CONSOLE}

uses
  SysUtils;
var
  n: int64;
  i: integer;
  sum: double;
begin
  readln (n);
  sum := 0;
  for i := 1 to n do begin
    sum := sum + sqrt (8) + i * 2;
  end;
  writeln (sum:0:5);
  readln;
  readln;
end.