You are given two arrays of integers a and b, which are both sorted in an ascending order and contain unique elements (i. e. no duplicates). You can take several (possibly zero) numbers from the array b and add them to a at any positions in any order. You want the array a to be an arithmetic progression after this. Your task is to find the maximal length of the resulting arithmetic progression represented by array a that can be achieved. If it is impossible to obtain an array forming an arithmetic progression, return -1.