```python
def drift_check(readings: list, step: int) -> int:
    if not readings or len(readings) < 2:
        return -1

    current_position = 0
    while True:
        next_position = position + step
        if next_position >= len(readings):
            break
        if readings[current_position] != readings[next_position]:
            return current_position
        current_position = next_position

    return -1
```