------
## 最终结论
```python
def latestStep(arr, m):
    n = len(arr)
    bits = [0] * n  # Initialize a binary string of size n with all bits set to 0
    last_step = -1  # Variable to record the latest step with a group of ones of length m

    for i in range(n):
        bits[arr[i] - 1] = 1  # Set the bit at position arr[i] to 1

        # Check for groups of ones of length m
        count = 0  # Counter for the length of current group of ones
        for bit in bits:
            if bit == 1:
                count += 1  # Increment count if we find a '1'
            else:
                if count == m:  # If we found a group of exactly length m
                    last_step = i + 1  # Update the last step (i is 0-indexed)
                count = 0  # Reset count for the next group
        if count == m:  # Check at the end in case the last group ends at the end of the array
            last_step = i + 1

    return last_step  # Return the recorded last step number, or -1 if no group of length m was found
```