In an article about the Miller-Rabin primality test, in the example section it says: "Suppose we wish to determine if $n = 221$ is prime. We write $n − 1 = 220$ as $2^{2}\times 55$, so that we have $s = 2$ and $d = 55$."
My question is: What are the missing steps that enables us to go from $220$ to $2^2\times 55$?