BOJ_1708

#include <iostream>
#include <vector>
#include <stack>
#include <algorithm>
typedef struct {
	int x, y;
	double tan;
	int index;
}POINT;
int main()
{
	int vertex_max;
	std::cin >> vertex_max;
	std::vector<POINT> vector(vertex_max, { 0, });
	int min = 0;
	for (int vertex = 0; vertex < vertex_max; vertex++)
	{
		POINT p = { 0, 0, 0, vertex+1 };
		std::cin >> p.x >> p.y;
		vector[vertex] = p;
		if (vector[min].y > p.y || vector[min].y == p.y && vector[min].x < p.x)
			min = vertex;
	}
	POINT m = vector[min];
	for_each(vector.begin(), vector.end(), [&m, &vector](POINT p){static int cnt = 0; vector[cnt].tan = atan2(p.y - m.y, p.x - m.x);  cnt++; });
	sort(vector.begin(), vector.end(), [&m](POINT p, POINT q){return (p.tan-q.tan) ? p.tan < q.tan : (m.y==p.y && m.y ==q.y) ? p.x<q.x : sqrt(pow(m.y-p.y,2.) + pow(m.x -p.x,2.)) < sqrt(pow(m.y-q.y,2.)+pow(m.x-q.x,2.))  ; });
	std::stack <int> stack;
	vector.push_back(vector[0]);
	for_each(vector.begin(), vector.end(), [&vector, &stack](POINT p){
		static int cnt = 0;
		if (stack.size() < 2)
			stack.push(cnt);
		else
		{
			int temp = stack.top();
			POINT p2 = vector[temp]; stack.pop();
			POINT p1 = vector[stack.top()]; stack.push(temp);
			int point_cnt = cnt;
			POINT p3 = p;
			//std::cout << "compare with " << p.index << std::endl;
			while ((double)(p2.x - p1.x) * (p3.y - p1.y) - (double)(p2.y - p1.y) * (p3.x - p1.x) <=0 && point_cnt < vector.size()-1)
				p3 = vector[point_cnt++];
			if (cnt == vector.size()-1)
			{
				int p = 3;
			}
			else if (point_cnt == vector.size()-1)
			{
				p3 = p;
				do{
					//std::cout << "point output" << vector[stack.top()].index << std::endl;
					stack.pop();
					int temp = stack.top();
					p2 = vector[temp]; stack.pop();
					p1 = vector[stack.top()]; stack.push(temp);
				} while ((double)(p2.x - p1.x) * (p3.y - p1.y) - (double)(p2.y - p1.y) * (p3.x - p1.x) <= 0 && !stack.empty());
				//std::cout << "point input" << vector[cnt].index << std::endl;
				stack.push(cnt);
			}
			else if (point_cnt >cnt)
			{
				//std::cout << "point output" << vector[stack.top()].index << std::endl;
				stack.pop();
				stack.push (cnt);
				//std::cout << "point input " << vector[cnt].index << std::endl;
			}
			else
			{
				if ((double)(p2.x - p1.x) / (p2.y - p1.y) == (double)(p3.x - p1.x) / (p3.y - p1.y))
				{
					//std::cout << "point output" << vector[stack.top()].index << std::endl;
					stack.pop();
					stack.push(point_cnt);
					//std::cout << "point input" << vector[point_cnt].index << std::endl;
				}
				else
				{
					stack.push(point_cnt);
					//std::cout << "point input" << vector[point_cnt].index << std::endl;
				}
			}
		}
		cnt++;
	});
	
	while (1)
	{
		int temp = stack.top();
		POINT p1 = vector[temp]; stack.pop();
		POINT p2 = vector[stack.top()]; stack.push(temp);
		if (m.y == p1.y && p1.y == p2.y)
			stack.pop();
		else
			break;
	}

	std::cout << stack.size()  << std::endl;
	/*while (!stack.empty())
	{
		std::cout << vector[stack.top()].index<< ",";
		stack.pop();
	}*/
	return 0;
}