stack + front

stack = []
def simple_stack():
    while True:
        operation= input().split()
        if operation[0] == "push":
            stack.append(int(operation[1]))
            print("ok")
        elif operation[0] == "pop":
            if len(stack)!=0:
                print(stack.pop(0))
            else:
                print("error")
        elif operation[0] == "front":
            print(stack[0])
        elif operation[0] == "back":
            if len(stack)!=0:
                print(stack[-1])
            else:
                print("error")
        elif operation[0] == "size":
            print(len(stack))
        elif operation[0] == "clear":
            stack.clear()
            print("ok")
        elif operation[0] == "exit":
            print("bye")
            break


simple_stack()