Kth Missing Positive Number

Given an array `arr` of positive integers sorted in a **strictly increasing order**, and an integer `k`. Return _the_ `kth` _**positive** integer that is **missing** from this array._   **Example 1:** **Input:** arr = [2,3,4,7,11], k = 5 **Output:** 9 **Explanation: **The missing positive integers are [1,5,6,8,9,10,12,13,...]. The 5th missing positive integer is 9. ``` **Example 2:** **Input:** arr = [1,2,3,4], k = 2 **Output:** 6 **Explanation: **The missing positive integers are [5,6,7,...]. The 2nd missing positive integer is 6. ```   **Constraints:** - `1 <= arr.length <= 1000` - `1 <= arr[i] <= 1000` - `1 <= k <= 1000` - `arr[i] < arr[j]` for `1 <= i < j <= arr.length`   **Follow up:** Could you solve this problem in less than O(n) complexity?