```ts
/** The smallest difference between any two values. */
export function minGap(values: number[]): number {
  if (values.length < 2) {
    throw new Error("Array must contain at least two elements.");
  }
  let smallest = Number.MAX_SAFE_INTEGER;
  for (let i = 1; i < values.length; i += 1) {
    const gap = Math.abs(values[i] - values[i - 1]);
    if (gap < smallest) {
      smallest = gap;
    }
  }
  return smallest === Number.MAX_SAFE_INTEGER ? -1 : smallest;
}
```