Devu and good strings

A string str is called a good string if there does not exist three difference indices i, j, k such that i, j, k are consecutive three terms of an arithmetic progression and str[i] = str[j] = str[k]. You are given a string s and two integers K, A. You have to find the number of good strings t with alphabet size A (i.e. if A = 1, then string can only have a's and if A = 2, string can only contain '