import sys class ListNode: def __init__(self, val) -> None: self.val = val self.next = None class LinkedList: def __init__(self) -> None: self.head = None def pop(self): if self.head == None: print('error') else: print(self.head.val) self.head = self.head.next def top(self): if self.head == None: print('error') else: print(self.head.val) def push(self,val): if self.head == None: self.head = ListNode(val) else: newNode = ListNode(val) newNode.next = self.head self.head = newNode def is_space(string): for i in range(len(string)): if string[i] == ' ': return True return False def main(): input_data = sys.stdin.read() lines = input_data.strip().split('\n') time = int(lines[0]) linkedList = LinkedList() for i in range(1,time+1): if is_space(lines[i]): com, data = lines[i].split(' ') linkedList.push(int(data)) elif lines[i] == 'top': linkedList.top() elif lines[i] == 'pop': linkedList.pop() if __name__ == '__main__': main()