finding a^b^c^… mod m

Question

I would like to calculate:

a^bcd... mod m

Do you know any efficient way since this number

Answer 1

a^bc mod m = a^{bc mod n} mod m, where n = φ(m) Euler's totient function.

If m is prime, then n = m-1.

Edit: as Nabb pointed out, this only holds if a is coprime to m. So you would have to check this first.