Definition: In this task, you are given an input list A comprising of numbers and alphabets. You need to extract and sort the unique alphabets in the list. The alphabets in the input list will only be in lowercase. Return -1 if there is no alphabet in the input list.
Input: ['6515', 'n', 'k', 'l', '6295', '8519', '4879', '4273', 'i', '8735', '1817', 'u', '4271', 's', 'n', 'q', '2729', 'a', '9413', '2027']

a, i, k, l, n, q, s, u