#include <bits/stdc++.h>
using namespace std;
int t, n, m, k, l, r, x, y;
char op;
void solve() {
    while (cin >> n&&n) {
        stack<int> stk;
        while (n--) {
            cin >> op;
            if (op == 'P') {
                cin >> x;
                stk.push(x);
            } else if (op == 'O') {
                if (!stk.empty()) {
                    stk.pop();
                }
            } else if (op == 'A') {
                if (stk.empty()) {
                    cout << "E";
                } else {
                    cout << stk.top();
                }
                cout << "\n";
            }
        }
        cout << "\n";
    }
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    solve();
    return 0;
}