Count the Number of Fair Pairs

class Solution:
    def countFairPairs(self, nums: List[int], lower: int, upper: int) -> int:
        nums.sort()
        def binary_search (nums,val):
            left =  0
            right = len(nums)-1
            result = 0
            while left < right:
                sum = nums[left] + nums[right]
                if sum < val:
                    result += right - left
                    left += 1
                else:
                    right -= 1
            return result
        return  binary_search(nums,upper +1) - binary_search(nums,lower )