<think>
Okay, I need to implement the audit_ticket function. Let's start by understanding the problem.

The function needs to check a depot ticket in the format AB-1234-3. The ticket should be a string that exactly matches this format. If it doesn't, return "shape". If it does match but the check digit is wrong, return "check". Otherwise, return "ok".

First, I need to validate the structure of the ticket. The format is two uppercase letters, followed by a hyphen, four digits, another hyphen, and one check digit. So the total length should be 9 characters. Let me check that. AB-1234-3 is 9 characters: 2 letters, 1 hyphen, 4 digits, 1 hyphen, 1 check digit. So the ticket must be exactly 9 characters long. Also, the parts must be in the correct positions.

So the first step is to check the shape. If the ticket is not exactly 9 characters, or if the parts don't match the required format, return "shape".

Next, check the check digit. The check digit is the last character. To compute the correct check digit, we need to sum the values of the two letters (A=1, B=2, ..., Z=26) and then sum each of the four digits multiplied by their position (1-4). Then, the check digit is the last digit of that total.

So, for example, AB-1234-3: letters A and B sum to 1 + 2 = 3. The digits 1, 2, 3, 4: 1*1 + 2*2 + 3*3 + 4*4 = 1 +4 +9 +16 = 30. Total sum is 3 + 30 = 33. The last digit is 3, which matches the check digit. So it's valid.

So the steps are:

1. Check if the ticket is exactly 9 characters long. If not, return "shape".
2. Check that the first two characters are uppercase letters, the third is '-', the next four are digits, the sixth is '-', and the last is a digit.
3. If the structure is correct, compute the check digit.
4. Compare the computed check digit with the actual one. If they match, return "ok", else "check".

Now, how to implement this in Python.

First, the function audit_ticket takes a string ticket. We need to check the structure.

Let's start with the shape check. The ticket must be exactly 9 characters. So if len(ticket) != 9, return "shape".

Then, check the format. The first two characters must be uppercase letters. The third must be '-'. The fourth to seventh must be digits. The eighth must be '-', and the ninth must be a digit.

So, for example, ticket[2] should be '-', ticket[5] should be '-', and the rest should be letters and digits.

So, for the first two characters: check that they are uppercase letters. We can use isalpha() and check that they are in A-Z.

For the next four digits: check that they are digits. So ticket[3:7] should be all digits.

Then, the last character must be a digit.

So, in code:

if len(ticket) != 9:
    return "shape"

if not (ticket[0].isalpha() and ticket[1].isalpha() and ticket[2] == '-' and ticket[3].isdigit() and ticket[4].isdigit() and ticket[5].isdigit() and ticket[6