题解 | #【模板】栈#

# 栈 
# Push  将x加入栈，保证x为int整数
# pop 输出栈顶  并让栈定出栈
# top 输出栈顶 栈顶不出栈
 
ipt_num = int(input()) # 输入一个数字（input输入的都是字符串类型），强制转成整型
list_num = []  # 栈
for i in range(ipt_num):
    action = input()
    if action.split()[0] == "push":  # 字符串分割
        num = action.split()[1]
        list_num.append(num)
    elif action == "pop":
        if list_num:
            print(list_num.pop())
        else:
            print("error")
    elif action == "top":
        if list_num:
            print(list_num[-1])
        else:
            print("error")