node* FindBeginning(node* head) {
	node* p1 = head;
	node* p2 = head;
	while (p2->next != nullptr) {
		if (p1 == p2) break;
		p1 = p1->next;
		p2 = p2->next->next;
	}
	if (p2->next == nullptr) return nullptr; // there is no loop

	p1 = head;
	while (p1 != p2) {
		p1 = p1->next;
		p2 = p2->next;
	}
	return p1;


}