```python
def min_gap(values: list) -> int:
    """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])
        
        # Always compare the current value with its previous one
        if gap < smallest:
            smallest = gap
            
    return smallest
```