You want to build `n` new buildings in a city. The new buildings will be built in a line and are labeled from `1` to `n`.

However, there are city restrictions on the heights of the new buildings:

- The height of each building must be a non-negative integer.
- The height of the first building must be `0`.
- The height difference between any two adjacent buildings cannot exceed `1`.

Additionally, there are city restrictions on the maximum height of specific buildings. These restrictions are given as a 2D integer array `restrictions` where `restrictions[i] = [idi, maxHeighti]` indicates that building `idi` must have a height less than or equal to `maxHeighti`.

It is guaranteed that each building will appear at most once in `restrictions`, and building `1` will not be in `restrictions`.

Return the maximum possible height of the tallest building.

Example 1:

    Input: n = 5, restrictions = [[2,1],[4,1]]
    Output: 2
    Explanation: The green area in the image indicates the maximum allowed height for each building.
    We can build the buildings with heights [0,1,2,1,2], and the tallest building has a height of 2.

Example 2:

    Input: n = 6, restrictions = []
    Output: 5
    Explanation: The green area in the image indicates the maximum allowed height for each building.
    We can build the buildings with heights [0,1,2,3,4,5], and the tallest building has a height of 5.

Example 3:

    Input: n = 10, restrictions = [[5,3],[2,5],[7,4],[10,3]]
    Output: 5
    Explanation: The green area in the image indicates the maximum allowed height for each building.
    We can build the buildings with heights [0,1,2,3,3,4,4,5,4,3], and the tallest building has a height of 5.

Constraints:

- `2 <= n <= 109`
- `0 <= restrictions.length <= min(n - 1, 105)`
- `2 <= idi <= n`
- `idi` is unique.
- `0 <= maxHeighti <= 109`