#include <bits/stdc++.h>
using namespace std;
const int MAXN=1e6+5,MAXB=2e7;
#define ll long long
int a[10001],l=1,r=0;
int main() {
	int n;
	cin>>n;
	while(n--){
		int f,x;
		cin>>f;
		if(f==1){
			cin>>x;
			a[++r]=x;
		}if(f==2){
			if(l-r>=0){
				l++;
			}else{
				cout<<"ERR_CANNOT_POP"<<endl;
			}
		}if(f==3){
			if(l-r>=0)
				cout<<a[l]<<endl;
			else
				cout<<"ERR_CANNOT_QUERY"<<endl;
		}if(f==4){
			cout<<l-r+1<<endl;	
		}
	}
	return 0;
}