how to calculate order cyclic multiplicative group of quadratic residues.

Let N = pq be one publicly known RSA modulus, in which p = 2p′ + 1, q = 2q′ + 1 are two large primes. p′ and q′ are also primes. All the quadratic residues modulo N forms a multiplicative cyclic group of order p′q′. My question is , how is this order computed?