#include <iostream>
#include <cstdio>
using namespace std;
const int N  =1E5 + 10;
int stk[N], tt;
int main(){
    int n;  scanf("%d", &n);
    while(n -- ){
        string op;  cin >> op;
        if(op == "push"){
            int x;  scanf("%d", &x);
            stk[tt ++ ] = x;
        }else if(op == "pop")   tt -- ;
        else if(op == "query")  cout << stk[tt - 1] << endl;
        else{
            if(tt == 0) cout << "YES" << endl;
            else    cout << "NO" << endl;
        }
    }
    return 0;
}