10
-2 1
2 -2
-1 -2
0 0
2 -1
0 -2
2 2
0 2
-1 -1
1 -2
 40
-8 24
2 -1
1 -18
72 -70
5 -4
-308 436
-19 40
36 -35
-178 265
-1 2
-7 30
-1 0
3 -2
200 -285
17 -16
-35 74
0 -4
-86 106
-1 4
-7 6
0 1
-5 4
-2 3
6 -5
-4 5
181 -262
76 -118
0 0
-7 18
-58 104
-5 6
-6 12
-3 4
1 0
11 -10
-86 130
-3 6
153 -236
-183 270
-33 64
 5
0 -1
0 -2
0 -3
0 -4
0 -5
 10
0 1
-1 -1
1 1
-1 0
1 -1
-2 -1
-2 2
-2 0
0 -2
0 -1
 20
-1 18
-2 5
-5 4
2 -33
9 -18
0 0
11 -22
2 0
-1 2
-4 41
1 6
1 -2
6 -12
0 1
-3 6
3 -6
3 -8
-1 4
2 -5
1 0
 10
2 1
-1 0
-2 -1
-1 1
0 2
2 -2
0 0
-2 -2
0 -2
-2 1
 20
-3 -3
0 4
-3 1
1 1
-1 2
-4 4
3 -1
-3 0
0 2
4 0
2 3
2 4
4 -3
-4 3
-1 1
1 3
-2 4
1 -2
1 -1
3 0
 3
0 0
0 -1
0 1
 10
2 1
-1 1
0 0
-3 1
-2 -3
-1 -2
-1 -1
1 2
3 -2
0 -2
 3
3 3
1 2
1 1
 4
0 0
1 0
2 0
3 0
 9
1 1
1 0
0 1
0 0
-1 0
-1 1
-1 -1
1 -1
0 -1
 7
1 1
-1 -1
1 0
0 1
-1 0
0 -1
0 0

4
 57
 4
 5
 21
 4
 10
 1
 1
 0
 2
 8
 3

import java.util.*;


public class Krok_2_b {
	
	
	static class point implements Comparable<point>{
		
		int x; int y;
		public point(int x, int y){
			this.x =x; this.y = y;
		}
		
		public int compareTo(point o){
			if (this.x == o.x)
				return this.y - y;
			return this.x - o.x;
		}
		
		public int Xmid(int x){
			return (x + this.x)/2;
		}
		
		public int Ymid(int y){
			return (y + this.y)/2;
		}

		
		
	}
	
	public static boolean isMid(int x, int y){
		if (Math.abs(y + x) < 2) return false;
		return (y + x)%2 == 0;
	}
	
	public static void main(String args[]){
		Scanner in=new Scanner(System.in);
		int n = in.nextInt();
		point a[]=new point[n];
		for (int i = 0; i<n; i++) a[i] =new point(in.nextInt(), in.nextInt());
		Arrays.sort(a);
		long count = 0;
		for (int i = 0; i<=n-3; i++){
			//ystem.out.println(a[i].Xmid(a[i+2].x) + " " +a[i].Ymid(a[i+2].y));
			if (isMid(a[i+2].x, a[i].x) && a[i].Xmid(a[i+2].x) == a[i+1].x && isMid(a[i].y, a[i+2].y) && a[i].Ymid(a[i+2].y) == a[i+1].y){
				count++;
			}
		}
		System.out.println(count);
	}

}
