input: Write a function to find the missing number in a sorted list.
  assert find_missing_num([1, 2, 3, 5])==4
  assert find_missing_num([10, 11, 12, 13, 15])==14
  assert find_missing_num([21, 22, 23, 25, 26])==24
output: def find_missing_num(nums):
      for i in range(len(nums) - 1):
          if nums[i] + 1 != nums[i + 1]:
              return nums[i] + 1