Program nunta;

var i, n : integer;
    f1, f2, f3 : longint;


begin
    assign(input,'nunta.in');
    reset(input);
    assign(output,'nunta.out');
    rewrite(output);

    read(n);
    f1 := 1;
    f2 := 2;
    for i := 3 to n do
    begin
        f3 := f1+f2;
        f1 := f2;
        f2 := f3;
    end;
    writeln(f3);
    close(input);
    close(output);
end.