#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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