if __name__ == '__main__':
    size =int(input())
    room_num = list(map(int,input().split()))
    
    set_rn = set(room_num)
    
    for i in set_rn:
        room_num.remove(i)
    
    set_rn.symmetric_difference_update(room_num)
    
    print(int("".join(str(i) for i in set_rn)))