二分答案 60pts，求 hack

int n, a[N];
bool check(int x) {
	multiset<int> st(a + 1, a + n + 1);
	multiset<int> q1;
	vector<int> q2;
	while (!st.empty()) {
		int v = *st.begin(), p = -1;
		for (int i = v; i <= v + x - 1; i++) {
			if (st.find(i) == st.end()) {p = i - 1; break;}
		}
		if (p != -1) {
			for (int i = v; i <= p; i++) st.erase(st.find(i)), q2.emplace_back(i);
		} else {
			for (int i = v; i <= v + x - 1; i++) st.erase(st.find(i));
			q1.emplace(v + x - 1);
		}
	}
	sort(q2.begin(), q2.end());
	for (int i : q2) {
		auto it = q1.find(i - 1);
		if (it == q1.end()) return false;
		q1.erase(it), q1.emplace(i);
	} 
	return true;
}

void _main() {
	cin >> n;
	for (int i = 1; i <= n; i++) cin >> a[i];
	int l = 1, r = n, res = -1;
	while (l <= r) {
		int mid = (l + r) >> 1;
		if (check(mid)) l = mid + 1, res = mid;
		else r = mid - 1;
	} cout << res;
}