#include<cstdio>

 #include<algorithm>

 #include<cmath>

 #define maxn 50010

 #define eps 1e-6

 using namespace std;

 int dcmp(double x)

 {

     return fabs(x) < eps ?  : (x >  ?  : -);

 }

 struct Point

 {

     double x;

     double y;

     Point(double x = , double y = ):x(x), y(y) {}

 };

 typedef Point Vector;

 Vector operator + (Point A, Point B)

 {

     return Vector(A.x + B.x, A.y + B.y);

 }

 Vector operator - (Point A, Point B)

 {

     return Vector(A.x - B.x, A.y - B.y);

 }

 Vector operator * (Point A, double p)

 {

     return Vector(A.x * p, A.y * p);

 }

 Vector operator / (Point A, double p)

 {

     return Vector(A.x / p, A.y / p);

 }

 double dot(Point a,Point b)

 {

     return a.x*b.x+a.y*b.y;

 }

 double cross(Point a,Point b)

 {

     return a.x*b.y-a.y*b.x;

 }

 Vector nomal(Vector a)

 {

     double l=sqrt(dot(a,a));

     return Vector(-a.y/l,a.x/l);

 }

 struct line

 {

     Point p;

     Vector v;

     double ang;

     line() {}

     line(Point p,Vector v):p(p),v(v)

     {

         ang=atan2(v.y,v.x);

     }

     bool operator<(const line &t)const

     {

         return ang<t.ang;

     }

 };

 bool onleft(line l,Point p)

 {

     return (cross(l.v,p-l.p)>);

 }

 Point getintersection(line a,line b)

 {

     Vector u=a.p-b.p;

     double t=cross(b.v,u)/cross(a.v,b.v);

     return a.p+a.v*t;

 }

 int halfplanintersection(line *l,int n,Point *poly)

 {

     sort(l,l+n);

     int first,last;

     Point *p=new Point[n];

     line *q=new line[n];

     q[first=last=]=l[];

     for(int i=; i<n; i++)

     {

         while(first<last && !onleft(l[i],p[last-]))last--;

         while(first<last && !onleft(l[i],p[first]))first++;

         q[++last]=l[i];

         if(fabs(cross(q[last].v,q[last-].v))<eps)

         {

             last--;

             if(onleft(q[last],l[i].p))q[last]=l[i];

         }

         if(first<last)p[last-]=getintersection(q[last-],q[last]);

     }

     while(first<last && !onleft(q[first],p[last-]))last--;

     if((last-first )<=)return ;

     p[last]=getintersection(q[last],q[first]);

     int m=;

     for(int i=first; i<=last; i++)poly[m++]=p[i];

     return m;

 }

 Point p[maxn],poly[maxn],v[maxn];

 line l[maxn];

 int main()

 {

     int n;

     while(scanf("%d",&n)!=EOF)

     {

         for(int i=;i<n;i++)scanf("%lf%lf",&p[i].x,&p[i].y);

         for(int i=;i<=n/;i++) swap(p[i],p[n-i]);

         int left=,right=n-;

         int ans;

         while(left<=right)

         {

             int mid=(left+right)>>;

             for(int i=;i<n;i++)

             {

                 v[i]=p[(i+mid+)%n]-p[i];

                 l[i]=line(p[i],v[i]);

             }

             int m=halfplanintersection(l,n,poly);

             if(!m){ans=mid;right=mid-;}

             else left=mid+;

         }

         printf("%d\n",ans);

     }

 }