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If a is not divisible by p
n! will not be a multiple of p if n > p and p is prime.
a^(p-1) % p= 1
Dr^-1 = (Dr^-1) * (1)
Dr^-1 % p = ((Dr^-1) * (Dr^(p-1)) ) % p
Dr^-1 % p = (Dr^(p-2)) % p