The Leetcode file system keeps a log each time some user performs a change folder operation.

The operations are described below:

- `"../"` : Move to the parent folder of the current folder. (If you are already in the main folder, remain in the same folder).
- `"./"` : Remain in the same folder.
- `"x/"` : Move to the child folder named `x` (This folder is guaranteed to always exist).

You are given a list of strings `logs` where `logs[i]` is the operation performed by the user at the `ith` step.

The file system starts in the main folder, then the operations in `logs` are performed.

Return the minimum number of operations needed to go back to the main folder after the change folder operations.

Example 1:

    Input: logs = ["d1/","d2/","../","d21/","./"]
    Output: 2
    Explanation: Use this change folder operation "../" 2 times and go back to the main folder.

Example 2:

    Input: logs = ["d1/","d2/","./","d3/","../","d31/"]
    Output: 3

Example 3:

    Input: logs = ["d1/","../","../","../"]
    Output: 0

Constraints:

- `1 <= logs.length <= 103`
- `2 <= logs[i].length <= 10`
- `logs[i]` contains lowercase English letters, digits, `'.'`, and `'/'`.
- `logs[i]` follows the format described in the statement.
- Folder names consist of lowercase English letters and digits.