def find_missing_number(nums):
if not nums:
return -1

# 计算公差
diff = nums[1] - nums[0]

# 遍历列表
for i in range(1, len(nums)):
    if nums[i] - nums[i-1] != diff:
        return nums[i-1] + diff

# 没有找到缺失的数字
return -1

测试样例

list=eval(input())
print(find_missing_number(list))