#include <cstdio>#include <cmath>#include <vector>#include <algorithm>using namespace std;struct Point {double x, y;};typedef Point Vector;vector<Point> P;struct Line {Point P, Q;Vector v;};int dcmp(double x, double y=0){static const double EPS=1e-8;double d=x-y;if (d>-EPS && d<EPS) return 0;return d<0?-1:1;}Vector operator-(Vector a, Vector b){return (Vector){a.x-b.x,a.y-b.y};}double Cross(Vector a, Vector b){return a.x*b.y-a.y*b.x;}bool Left(Point P, const Line &L){return dcmp(Cross(L.P-P,L.Q-P))>0;}Point Intersection(const Line &a, const Line &b){double t=Cross(b.v,a.P-b.P)/Cross(a.v,b.v);return (Point){a.P.x+t*a.v.x, a.P.y+t*a.v.y};}bool Check(int x){int i, j, l, r, n=P.size();vector<Line> L(n);for (i=0; i<n; i++) {j = (i+x+1)%n;L[i].P = P[i];L[i].Q = P[j];L[i].v = P[j]-P[i];}vector<Point> P(n);vector<int> Q(n);Q[l=r=0] = 0;for (i=1; i<n; i++) {while (r>l && !Left(P[r-1],L[i])) r--;while (r>l && !Left(P[l],L[i])) l++;Q[++r] = i;if (dcmp(Cross(L[Q[r]].v,L[Q[r-1]].v)) == 0) {r--;if (Left(L[i].P, L[Q[r]])) Q[r]=i;}if (r>l) P[r-1]=Intersection(L[Q[r]],L[Q[r-1]]);}while (r>l && !Left(P[r-1],L[Q[l]])) r--;while (r>l && !Left(P[l],L[Q[r]])) l++;return r-l+1>=3;}int main(){int n, i;while (scanf("%d",&n)==1) {P.resize(n);for (i=0; i<n; i++)scanf("%lf%lf", &P[i].x, &P[i].y);reverse(P.begin(), P.end());int low=1, high=n-2, mid;while (low < high) {mid = (low+high)/2;if (Check(mid)) low=mid+1;else high=mid;}printf("%dn", low);}return 0;}
