Find n-th missing element in unsorted array

Suppose you're given an unsorted array, a. The array is intended to have all integer elements between its minimum and maximum, but could be missing some elements. Write a python function to find the n-th missing element in the array.

Consider the array in sorted order, then find the n-th missing number. If the n-th missing number does not exist, you can output 'Does not exist'.

Examples:

    
Input: a = [2, 3, 11, 9], n = 3
    
Output: 6
    

Missing elements in array: [4, 5, 6, 7, 8, 10]. So, the 3rd missing element is 6.

    
Input: a = [2, 3, 5], n = 5
    
Output: 'Does not exist'

Since there is no 5th missing element in the array, we output 'Does not exist' (the only missing element here is 4).