Given an array `arr` that represents a permutation of numbers from `1` to `n`.

You have a binary string of size `n` that initially has all its bits set to zero. At each step `i` (assuming both the binary string and `arr` are 1-indexed) from `1` to `n`, the bit at position `arr[i]` is set to `1`.

You are also given an integer `m`. Find the latest step at which there exists a group of ones of length `m`. A group of ones is a contiguous substring of `1`'s such that it cannot be extended in either direction.

Return the latest step at which there exists a group of ones of length exactly `m`. If no such group exists, return `-1`.

Example 1:

    Input: arr = [3,5,1,2,4], m = 1
    Output: 4
    Explanation:
    Step 1: "00100", groups: ["1"]
    Step 2: "00101", groups: ["1", "1"]
    Step 3: "10101", groups: ["1", "1", "1"]
    Step 4: "11101", groups: ["111", "1"]
    Step 5: "11111", groups: ["11111"]
    The latest step at which there exists a group of size 1 is step 4.

Example 2:

    Input: arr = [3,1,5,4,2], m = 2
    Output: -1
    Explanation:
    Step 1: "00100", groups: ["1"]
    Step 2: "10100", groups: ["1", "1"]
    Step 3: "10101", groups: ["1", "1", "1"]
    Step 4: "10111", groups: ["1", "111"]
    Step 5: "11111", groups: ["11111"]
    No group of size 2 exists during any step.

Constraints:

- `n == arr.length`
- `1 <= m <= n <= 105`
- `1 <= arr[i] <= n`
- All integers in `arr` are distinct.