#include <bits/stdc++.h>
using namespace std;
int a[10010], head = 0, tail = -1, n, b, c;

void push(int x) {
	a[++tail] = x;
}

void pop() {
	head++;
}

int query() {
	return tail - head + 1;
}

bool empty() {
	return head > tail;
}

int front() {
	return a[head];
}

int main() {
	cin >> n;
	for (int j = 0; j < n; j++) {
		cin >> b;
		if (b == 1) {
			cin >> c;
			push(c);
		} else if (b == 2) {
			if (empty() == 1) {
				cout << "ERR_CANNOT_POP\n";
			} else {
				pop();
			}
		} else if (b == 3) {
			if (empty() == 1) {
				cout << "ERR_CANNOT_QUERY\n";
			} else {
				cout << front() << endl;
			}
		} else if (b == 4) {
			cout << query() << endl;
		}
	}
	return 0;
}

#include <bits/stdc++.h> using namespace std; int a[10010], head = 0, tail = -1, n, b, c;

void push(int x) { a[++tail] = x; }

void pop() { head++; }

int query() { return tail - head + 1; }

bool empty() { return head > tail; }

int front() { return a[head]; }

int main() { cin >> n; for (int j = 0; j < n; j++) { cin >> b; if (b == 1) { cin >> c; push(c); } else if (b == 2) { if (empty() == 1) { cout << "ERR_CANNOT_POP\n"; } else { pop(); } } else if (b == 3) { if (empty() == 1) { cout << "ERR_CANNOT_QUERY\n"; } else { cout << front() << endl; } } else if (b == 4) { cout << query() << endl; } } return 0; }

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