What two letters best complete this sequence a y d v g s j p?

The two letters that best complete the sequence are 'f' and 'e'. The sequence consists of every third letter in the alphabet, starting from 'a' (a, d, g, j, m, p) and then moving backwards every third letter (y, v, s, p, m, j, f). Thus, the next letters in the pattern are 'f' and 'e'.