#include <stdio.h>


struct stack{
    int data;
    stack* prev;
    stack* top;
};

stack *top = NULL;

void push(int a){
    stack *p = new stack();
    p -> data = a;
    p -> prev = top;
    top = p;
    printf("ok\n");
}

int pop(){
	if(top == NULL){
		printf("error");
	}
    int ans = top -> data;
    top = top -> prev;
    return ans;
}

int size(){

	int count = 0;
	while(top != NULL){
		count++;
		top = top -> prev;
	}
	return count;
}

int back (){
	if(top == NULL){
		printf("error");
	}
	return top -> data;

}
void exit(){
	printf("bye");
}

void clear(){
	stack *top = NULL;
	printf("ok\n");
}

int main(){
	int num;
	char str[6];
	while(str[1] != 120){
		scanf("%s", str);
		if(str[1] == 'u'){
			scanf("%d", &num);
			push(num);
		}
		if(str[1] == 'o'){
			printf("%d\n", pop());
		}
		if(str[1] == 'i'){
			printf("%d\n", size());

		}
		if(str[1] == 'a'){
			printf("%d\n", back());
		}
		if(str[1] == 'l'){
			clear();
		}
		if(str[1] == 'x'){
			exit();
		}
	}
}