题解 | #【模板】栈#

from pickle import NONE
# import sys

# for line in sys.stdin:
#     a = line.split()
#     print(int(a[0]) + int(a[1]))
class Stack:
    def __init__(self) -> None:
        self.stack = []

    def push(self,x) -> None:
        self.stack.append(x)

    def pop(self) -> None:
        if len(self.stack) != 0:
            print(self.stack.pop())
        else:
            print('error')

    def top(self) -> None:
        if len(self.stack) != 0:
            print(self.stack[-1])
        else:
            print('error')

    # def is_empty(self):
    #     return len(self.stack) == 0

stack = Stack()
n=int(input())
for i in range(n):
    a = [i for i in input().split()]
    if len(a) == 2:
        b = int(a[1])
        stack.push(b)
    else:
        if a[0] == 'pop':
            stack.pop()
        else:
            stack.top()