How many sequences with a maximum of four signs of the Morse alphabet can be formed?

Question

I have a statement that says:

The Morse alphabet uses the signs . and - .Using a maximum of four these signs, how many different sequences can you form?

So, the order matters, because -. is different of .-, then I use the formula of variance (with repetition): $n^m$, with $n = 2$ (symbols of Morse), $m = 4$ (maximum), so my result is: $2^4 = 16$, but the correct result must be $30$, what is wrong with my development?

Answer 1

You counted the sequences with exactly four signs. You were expected to also count the sequences with one to three signs as the question asks about at most four signs.

Answer 2

Every symbol can be used in each of the four positions. $2^4$

Every symbol can be used in each of three positions: $2^3$

Every symbol can be used in each of two positions: $2^2$

Every symbol can be used in one position: $2$

Sum.