For p, q ∈ R, consider the real valued function f(x) = (x - p)2 -q, x ∈ R and q > 0. Let a1 , a2 , a3 , and a4 be in an arithmetic progression with mean p and positive common difference. If |f(a1)| = 500 for all i = 1, 2, 3, 4, then the absolute difference between the roots of f(x) = 0 is___.