#include<bits/stdc++.h>

using namespace std;

const int N = 100005;

int st[N], t;

int main(){
    int n;
    cin>>n;
    while(n--){
        string op;
        int x;
        cin>>op;
        if(op == "push"){
            cin>>x;
            st[t++] = x;
        }
        else if(op == "pop"){
            t--;
        }
        else if(op == "query"){
            cout<<st[t-1]<<endl;
        }
        else if(op == "empty"){
            cout<<(t ? "NO" : "YES")<<endl;
        }
    }
    return 0;
}