from collections import deque
d=deque()
f= open('input.txt')
for s in f:
    s=s.split()
    if s[0]=='push_front':
        d.appendleft(s[1])
        print('ok')
    if s[0]=='push_back':
        d.append(s[1])
        print('ok')
    if s[0]=='pop_front':
        if len(d)>0:
            print(d.popleft())
        else:
            print('error')
    if s[0]=='pop_back':
        if len(d)>0:
            print(d.pop())
        else:
            print('error')
    if s[0]=='front':
        if len(d)>0:
            print(d[0])
        else:
            print('error')
    if s[0]=='back':
        if len(d)>0:
            print(d[len(d)-1])
        else:
            print('error')
    if s[0]=='size':
        print(len(d))
    if s[0]=='clear':
        d.clear()
        print('ok')
    if s[0]=='exit':
        print('bye')
        break