Evenly Spaced Trees

**English**RU

You need to add some trees and keep them evenly spaced.

You are given a list of integers as an input value. This is the position of an existing tree. You must return the minimum number of additional trees needed so that they could be evenly spaced.

- Positions of the existing trees are already sorted.
- All positions of trees are integers.

**Example:**

evenly_spaced_trees([0, 2, 6]) == 1 # add to 4. evenly_spaced_trees([1, 3, 6]) == 3 # add to 2, 4 and 5. evenly_spaced_trees([0, 2, 4]) == 0 # no need to add.

**Input: **
The position of the existing trees (a list of integers).

**Output: **
The minimum number of additional trees (an integer).

**How it is used:**

Landscape design.

**Precondition:**

- 0 ≤ Position of tree ≤ 100
- 3 ≤ The existing trees