#include <iostream>
#include <fstream>
//#include <conio.h>
#include <cstdlib>
#include <math.h>
using namespace std;
struct Punct{
       double x;
       double y;
       };

int main()
{
    Punct p[1500];
    int n,k=0;
    double l1,l2,l3;
    ifstream myfile1 ("triang.in");
    ofstream myfile2 ("triang.out");
   if (myfile1.is_open())
   {
   while (! myfile1.eof())
    {
     myfile1>>n;
     //Punct p[];
     
     for(int i=0; i<n; i++)
     {     
         myfile1>>p[i].x >>p[i].y; 
          
     }    
    }
    myfile1.close();
   }
   
for(int i=0; i<n; i++)
     {
          l1=sqrt((p[i].x-p[i+1].x)*(p[i].x-p[i+1].x)+(p[i].y-p[i+1].y)*(p[i].y-p[i+1].y)); 
          l2=sqrt((p[i].x-p[i+2].x)*(p[i].x-p[i+2].x)+(p[i].y-p[i+2].y)*(p[i].y-p[i+2].y)); 
          l3=sqrt((p[i+2].x-p[i+1].x)*(p[i+2].x-p[i+1].x)+(p[i+2].y-p[i+1].y)*(p[i+2].y-p[i+1].y)); 
          if(l1==l2==l3)k++;
             }
myfile2<<k;
myfile2.close();
//getch();
   
return 0;
}