Split all numbers into several groups, with each group being an arithmetic sequence with a common difference of k.

How many K-free subsets are there for each group? This can be solved by dp: dp[i] = dp[i-1] + dp[i-2], meaning if we choose ith element, we cannot choose (i-1)th; otherwise we can choose (i-1)th element.

After solving the problem for every group, the final result is just the product of the sub-problems.