def missing_number(items: list[int]) -> int:
    # your code here
    n = len(items)
    total = (n + 1)*(n + 2)/2
    arr_sum = sum(items)
    return total - arr_sum

Where am I going wrong in this? for some reason my answer is not right and the mission is not being completed. Also, follow me and i'll follow back, and we can connect! Trying to get to know more people here, Thanks!