Let $ A = \{1, 2, 3, ..., 10\} $ and $ R $ be a relation on $ A $ such that $ R = \{(a, b) : a = 2b + 1\} $. Let $ (a_1, a_2), (a_3, a_4), (a_5, a_6), ..., (a_k, a_{k+1}) $ be a sequence of $ k $ elements of $ R $ such that the second entry of an ordered pair is equal to the first entry of the next ordered pair. Then the largest integer $ k $, for which such a sequence exists, is equal to: