d = []
head = 0
size = 0
tail = 0
data = 0


def push_front(data):
    d.insert(head, data)
    size += 1


def push_back(data):
    d.append(data)
    size += 1
    tail += 1


def pop_front():
    p = d[head]
    head += 1
    size -= 1
    return p


def pop_back():
    p = p[tail]
    tail -= 1
    size -= 1
    return p


def front():
    return d[head]


def back():
    return d[tail]



f = open('input.txt')
for line in f:
    s = line.split()
    if (s[0] == 'push_front'):
        push_front(s[1])
        print('ok')
    if (s[0] == 'push_back'):
        push_back(s[1])
        print('ok')
    if (s[0] == 'pop_front'):
        if (size > 0):
            data = pop_front()
            print(data)
        else:
            print('error')
    if (s[0] == 'pop_back'):
        if (size > 0):
            data = pop_back()
            print(data)
        else:
            print('error')
    if (s[0] == 'front'):
        if (size > 0):
            data = front()
            print(data)
        else:
            print('error')
    if (s[0] == 'back'):
        if (size > 0):
            data = back()
            print(data)
    if (s[0] == 'size'):
        print(size)
    if (s[0] == 'clear'):
        head = 0
        tail = 0
        size = 0
        print('ok')
    if (s[0] == 'exit'):
       print('bye')
       exit