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.