题解 | #【模板】栈#

from os import X_OK
import sys


#from numpy import number
#for line in sys.stdin:
#    a = line.split()
#    print(int(a[0]) + int(a[1]))

#定义栈类，将push,top,和pop函数放入到栈类中
class Mystack():
    def __init__(self):
        self.val=0
        self.L=[]

    def push(self,x):
        self.L.append(x)
        self.val

    def top(self):
        x=self.L[-1]
        return x

    def pop(self):
        x=self.L.pop()
        return x

	  
	  #接收操作的个数
n = int(input())



#初始化栈对象
stack=Mystack()


#将操作放入到Oprations操作列表中
opration =[]
while n > 0:
    s = input()
    opration.append(s)
    n-=1
	
	
	#遍历操作列表，分为三种情况
for x in opration:
    if x[0:4]=="push":
        number=int(x[4:])
        stack.push(number)
    
    if x=="pop":
        try:
            print(stack.pop())
        except:
            print("error")

    if x=="top":
        try:
            print(stack.top())
        except:
            print("error")