n = int(input())
res = []
for i in range(n):
    operate = input()
    if operate == '2':
        if res:
            del res[0]
        else:
            print('ERR_CANNOT_POP')
    elif operate == '3':
        print(res[0] if res else 'ERR_CANNOT_QUERY')
    elif operate == '4':
        print(len(res))
    else:
        value = operate.split(' ')
        res.append(value[-1])