```ts
function driftCheck(readings: number[], step: number): number {
    if (readings.length < 2) return -1;

    for (let i = 0; i < readings.length - 1; i++) {
        const diff = Math.abs(readings[i] - readings[i + 1]);
        if (diff > step) return i;
    }

    return -1;
}
```