You are given a list of `preferences` for `n` friends, where `n` is always even.

For each person `i`, `preferences[i]` contains a list of friends sorted in the order of preference. In other words, a friend earlier in the list is more preferred than a friend later in the list. Friends in each list are denoted by integers from `0` to `n-1`.

All the friends are divided into pairs. The pairings are given in a list `pairs`, where `pairs[i] = [xi, yi]` denotes `xi` is paired with `yi` and `yi` is paired with `xi`.

However, this pairing may cause some of the friends to be unhappy. A friend `x` is unhappy if `x` is paired with `y` and there exists a friend `u` who is paired with `v` but:

- `x` prefers `u` over `y`, and
- `u` prefers `x` over `v`.

Return the number of unhappy friends.

Example 1:

    Input: n = 4, preferences = [[1, 2, 3], [3, 2, 0], [3, 1, 0], [1, 2, 0]], pairs = [[0, 1], [2, 3]]
    Output: 2
    Explanation:
    Friend 1 is unhappy because:
    - 1 is paired with 0 but prefers 3 over 0, and
    - 3 prefers 1 over 2.
    Friend 3 is unhappy because:
    - 3 is paired with 2 but prefers 1 over 2, and
    - 1 prefers 3 over 0.
    Friends 0 and 2 are happy.

Example 2:

    Input: n = 2, preferences = [[1], [0]], pairs = [[1, 0]]
    Output: 0
    Explanation: Both friends 0 and 1 are happy.

Example 3:

    Input: n = 4, preferences = [[1, 3, 2], [2, 3, 0], [1, 3, 0], [0, 2, 1]], pairs = [[1, 3], [0, 2]]
    Output: 4

Constraints:

- `2 <= n <= 500`
- `n` is even.
- `preferences.length == n`
- `preferences[i].length == n - 1`
- `0 <= preferences[i][j] <= n - 1`
- `preferences[i]` does not contain `i`.
- All values in `preferences[i]` are unique.
- `pairs.length == n/2`
- `pairs[i].length == 2`
- `xi != yi`
- `0 <= xi, yi <= n - 1`
- Each person is contained in exactly one pair.