#include <string>
#include <iostream>
using namespace std;

const int maxn = 100010;
int st[maxn], top;

int main(int argc, char const *argv[])
{
    int m; cin >> m;
    string op;

    while (m--) {
        cin >> op;

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