==== SOLUTION ====

“`python
def mid(s):
     length = len(s)
     if length % 2 == 0: # If the length of the string is even
           return “”
     else:
           middle_index = length // 2
           return s[middle_index]

# Test the function
print(mid(“abc”)) # Output: “b”
print(mid(“aaaa”)) # Output: “”
“`

Explanation:

1. `def mid(s):`: This line defines a function named `mid` that takes a string `s` as its parameter.

2. `length = len(s)`: This line calculates the length of the string `s` and stores it in the variable `length`.

3. `if length % 2 == 0:`: This `if` statement checks if the length of the string is even by checking if the length modulo 2 equals 0. If the length is even, it means there is no middle letter, so the function returns an empty string (`””`).

4. `else:`: If the length of the string is odd, execution moves to this `else` block.

5. `middle_index = length // 2`: This line calculates the index of the middle letter by dividing the length of the string by 2 using floor division (`//`), which discards the remainder.

6. `return s[middle_index]`: This line returns the character at the calculated middle index of the string `s`.

So, the function returns the middle letter of the string if the length is odd, and an empty string otherwise.