def push(A, n):
    A.append(n)
    print('ok')
def pope(A):
    if len(A) == 0:
        print('error')
        return None
    print(A[0])
    A.pop(0)
def size(A):
    print(len(A))
def front(A):
    if len(A) == 0:
        print('error')
        return None
    print(A[0])
def clear(A):
    A.clear()
    print('ok')

A = []
inp = ' '
while inp[0] != 'exit':
    inp = input().split()
    if inp[0] == 'push':
        push(A, inp[1])
    elif inp[0] == 'pop':
        pope(A)
    elif inp[0] == 'size':
        size(A)
    elif inp[0] == 'front':
        front(A)
    elif inp[0] == 'clear':
        clear(A)
print('bye')