On a table are N cards, with a positive integer printed on the front and back of each card (possibly different). We flip any number of cards, and after we choose one card. If the number X on the back of the chosen card is not on the front of any card, then this number X is good. What is the smallest number that is good? If no number is good, output 0. Here, fronts[i] and backs[i] represent the number on the front and back of card i. A flip swaps the front and back numbers, so the value on the front is now on the back and vice versa. Example: Input: fronts = [1,2,4,4,7], backs = [1,3,4,1,3] Output: 2 Explanation: If we flip the second card, the fronts are [1,3,4,4,7] and the backs are [1,2,4,1,3]. We choose the second card, which has number 2 on the back, and it isn't on the front of any card, so 2 is good. Note: 1 <= fronts.length == backs.length <= 1000. 1 <= fronts[i] <= 2000. 1 <= backs[i] <= 2000.