class Node:
    def __init__(self, data):
        self.data = data
        self.prev = None
        self.next = None


def create_list(arr):
    if not arr:
        return None
    head = Node(arr[0])
    cur = head
    for x in arr[1:]:
        n = Node(x)
        cur.next = n
        n.prev = cur
        cur = n
    return head


def print_list(head):
    res = []
    while head:
        res.append(str(head.data))
        head = head.next
    print("Список:", " ".join(res) if res else "null")


def remove_odd_positions(head):
    index = 1
    cur = head
    new_head = None
    last = None
    while cur:
        nxt = cur.next
        if index % 2 == 0:
            if not new_head:
                new_head = cur
                cur.prev = None
            else:
                last.next = cur
                cur.prev = last
            last = cur
        index += 1
        cur = nxt
    if last:
        last.next = None
    return new_head


while True:
    try:
        print("\n1 - Ввести список\n0 - Выход")
        cmd = input("Выбор: ")
        if cmd == "0":
            break
        if cmd == "1":
            arr = list(map(int, input("Введите числа: ").split()))
            head = create_list(arr)
            head = remove_odd_positions(head)
            print_list(head)
    except:
        print("Ошибка ввода")