Find missing value in sorted consecutive array

Given a sorted array `nums` of **distinct integers**. The numbers are supposed to form a consecutive sequence starting from some unknown integer `start`: - If nothing is missing, the sequence would be: `start, start+1, start+2, ..., start+n-1` where `n = len(nums)`. - But **at most one** number from this sequence may be missing in `nums`. Return the missing number if one exists; otherwise return `null` (or `-1`, but be consistent). **Requirements** - Time complexity must be **O(log n)**. - Space complexity must be **O(1)**. **Examples** - `nums = [4,5,6,8,9]` → missing is `7` - `nums = [-3,-2,-1,0,1]` → no missing → return `null` - `nums = [10,11,12,13,15]` → missing is `14` **Notes** - `start` can be any integer (including negative). - `nums` is already sorted ascending and contains no duplicates.