rot(C, B, N) ->
    (C - B + N) rem 26 + B.

encrypt([], _) -> [];
encrypt([C | S], N) ->
    Nc = if
        ($A =< C) andalso ($Z >= C) ->
            rot(C, $A, N);
        ($a =< C) andalso ($z >= C) ->
            rot(C, $a, N);
        true ->
            C
    end,
    [Nc | encrypt(S, N)].

decrypt(String, N) ->
    encrypt(String, 26-N).

test_version() -> 1.