function at window

struct min_window {
	deque <pr<int,int>> a;
	void add_elem(int i, int zn) {
		while (len(a) && a.back().second > zn) {
			a.pop_back();
		}
		a.push_back({ i, zn });
	}
	int get_min() {
		return a[0].second;
	}
	void pop_elem(int i) {
		if (len(a) && a[0].first == i) a.pop_front();
	}
};