출처 | https://www.acmicpc.net/problem/18258
import sys
from collections import deque
input = sys.stdin.readline
n = int(input())
queue = deque([])
for i in range(n):
command = input().split()
if command[0] == "push":
queue.append(command[1])
if command[0] == "pop":
if len(queue) > 0:
print(queue.popleft())
else:
print(-1)
if command[0] == "size":
print(len(queue))
if command[0] == "empty":
if len(queue) == 0:
print(1)
else:
print(0)
if command[0] == "front":
if len(queue) > 0:
print(queue[0])
else:
print(-1)
if command[0] == "back":
if len(queue) > 0:
print(queue[-1])
else:
print(-1)