Leetcode 2310. Sum of Numbers With Units Digit K

Find the minimum number of positive integers (repetition allowed) whose units digit is k that sum to num, or return -1 if impossible. The problem reduces to a modular-arithmetic check: you need the smallest m (possibly 0 when num==0) such that (m*k) % 10 == num % 10 and m*k <= num, otherwise it's impossible.