#include<bits/stdc++.h>
using namespace std;
stack<int> a;
int main()
{
    int n,Cmd,num;
    scanf("%d",&n);
    while(n--)
    {
        scanf("%d",&Cmd);
        if(Cmd==1)
        {
            scanf("%d",&num);
            a.push(num);
        }
        if(Cmd==2)
        {
            if(!a.empty())
            {
                a.pop();
            }
            else
            {
                cout<<"ERR_CANNOT_POP\n";
            }
        }
        if(Cmd==3)
        {
            if(!a.empty())
            {
                cout<<a.top()<<"\n";
            }
            else
            {
                cout<<"ERR_CANNOT_QUERY\n";
            }
        }
        if(Cmd==4)
        {
            if(a.empty()) cout<<"0\n";
            else cout<<a.size()<<"\n";
        }
    }
    return 0;
}