var n,a,b,aux:longint;
procedure citire;
          begin
               assign(input,'nunta.in');
               reset(input);
               read(input,n);
               close(input)
          end;
procedure gasire;
          var i:integer;
          begin if n=1 then b:=1
                else if n=2 then b:=2
                else if b=3 then b:=3
                else begin
                a:=3;
                b:=5;
                for i:=3 to n do begin
                    aux:=a+b;
                    a:=b;
                    b:=aux
                    end
                    end
          end;
procedure afisare;
          begin
                assign(output,'nunta.out');
                rewrite(output);
                write(output,b);
                close(output)
          end;
begin
     citire;
     gasire;
     afisare
end.