go = True
while(go):
    values = list(map(int, input().strip().split(" ")))
    if all([v == 0 for v in values]):
        go = False
    else:
        total_num_of_members, p_members, r_members = values
        vote_need_number = (total_num_of_members//2)+1
        person_need_number = max(vote_need_number-p_members,0)
        remaining_members = total_num_of_members-p_members-r_members
        if person_need_number>remaining_members:
            person_need_number = -1
        print(person_need_number)
            
cdc8208bfc5936a911348eaab66b53801aec2f76
<mask token>

<mask token>
while go:
    values = list(map(int, input().strip().split(' ')))
    if all([(v == 0) for v in values]):
        go = False
    else:
        total_num_of_members, p_members, r_members = values
        vote_need_number = total_num_of_members // 2 + 1
        person_need_number = max(vote_need_number - p_members, 0)
        remaining_members = total_num_of_members - p_members - r_members
        if person_need_number > remaining_members:
            person_need_number = -1
        print(person_need_number)

go = True
while go:
    values = list(map(int, input().strip().split(' ')))
    if all([(v == 0) for v in values]):
        go = False
    else:
        total_num_of_members, p_members, r_members = values
        vote_need_number = total_num_of_members // 2 + 1
        person_need_number = max(vote_need_number - p_members, 0)
        remaining_members = total_num_of_members - p_members - r_members
        if person_need_number > remaining_members:
            person_need_number = -1
        print(person_need_number)

go = True
while(go):
    values = list(map(int, input().strip().split(" ")))
    if all([v == 0 for v in values]):
        go = False
    else:
        total_num_of_members, p_members, r_members = values
        vote_need_number = (total_num_of_members//2)+1
        person_need_number = max(vote_need_number-p_members,0)
        remaining_members = total_num_of_members-p_members-r_members
        if person_need_number>remaining_members:
            person_need_number = -1
        print(person_need_number)
            