An ordered sequence of numbers from 1 to N is given. One number might have deleted from it, then the remaining numbers were mixed. Find the number that was deleted.
Example: 
 - The starting array sequence is `[1,2,3,4,5,6,7,8,9]`
 - The mixed array with one deleted number is `[3,2,4,6,7,8,1,9]`
 - Your function should return the int `5`.
If no number was deleted from the array and no difference with it, your function should return the int `0`.
Note that N may be 1 or less (in the latter case, the first array will be `[]`).
def find_deleted_number(arr, mixed_arr):
    return 0 if not arr else arr[-1]*(arr[-1]+1)/2 - sum(mixed_arr)
def find_deleted_number(arr, mixed_arr):
    return 0 if not arr else arr[-1]*(arr[-1]+1)/2 - sum(mixed_arr)