Program Nunta;
var n : integer;
    rez : int64;

procedure Citeste;
var Intrare : text;
begin
  assign(Intrare,'nunta.in');
  reset(Intrare);
  readln(Intrare,n);
  close(Intrare);
end;

procedure Calculeaza;
var i : integer;
    no,nv,mo,mv : int64;
begin
  no:=0;
  nv:=1;
  for i:=2 to n do
   begin
     mo:=no;
     mv:=nv;
     no:=mv;
     nv:=mo+mv;
   end;
  rez:=no+nv;
end;

procedure Scrie;
var Iesire : text;
begin
  assign(Iesire,'nunta.out');
  rewrite(Iesire);
  write(Iesire,rez);
  close(Iesire);
end;

begin
  Citeste;
  Calculeaza;
  Scrie;
end.