#include <bits/stdc++.h>

using namespace std;
const int N = 100010;
int stk[N], tt;

int main() {
    string op;
    int n;
    cin >> n;
    while (n--) {
        cin >> op;
        if (op == "push") {
            int x;
            cin >> x;
            stk[++tt] = x;
        } else if (op == "pop")
            tt--;
        else if (op == "query")
            printf("%d\n", stk[tt]);
        else if (op == "empty") {
            if (tt == 0) printf("YES\n");
            else printf("NO\n");
        }
    }
    return 0;
}