tourist_rooms = list(map(int,input().split()))

captain_room =None

if group_size in range(1,1000):
    unique_rooms = list(set(tourist_rooms))
    for room in unique_rooms:
        tourist_rooms.pop(tourist_rooms.index(room))
        if room not in tourist_rooms:
            captain_room = room
        else:
            pass
            
print(captain_room)