Position X28180
Write a function @pos_a(s, k)@ that takes a string s and a non-negative integer k and returns the position where the k-th letter a occurs within s, or -1 if the string s contains fewer than k letters a.
Sample session
>>> pos_a('', 1) -1 >>> pos_a('hola', 2) -1 >>> pos_a('ara', 3) -1 >>> pos_a('lalaland', 3) 5 >>> pos_a('almendro', 1) 0