```python
def list_gap(values:       """The smallest difference between any two values."""
    if len(values) < 2:
        return -1
    smallest = abs(values[1] - values[0])
    for i in range(1, len(values)):
        gap = abs(values[i] - values[i - 1])
        if gap < smallest:
            smallest = gap
    return smallest
```