#include<bits/stdc++.h>
using namespace std;
const int N=100010;
int a[N],l=0;
int main(){
	int n;
	cin>>n;
	for(int j=1;j<=n;j++){
		int x;
		cin>>x;
		if(x==1){
			int y;
			l++;
			cin>>y;
			a[l]=y;
		}
		if(x==2){
			if(l==0){
				cout<<"ERR_CANNOT_POP"<<endl;
				continue;
			}
			for(int i=1;i<=l;i++)
				a[i]=a[i+1];
			l--;
		}
		if(x==3){
			if(l==0){
				cout<<"ERR_CANNOT_POP"<<endl;
				continue;
			}
			cout<<a[1]<<endl;
		}
		if(x==4){
			cout<<l<<endl;
		}
	} 
	return 0;
}

不是到底是哪里有问题呀