d.cpp

#include<algorithm>
#include<iostream>
#include<cstring>
using namespace std;

template<class T>
struct point{
  T x,y;
  point &operator+=(const point &a){ x+=a.x; y+=a.y; }
  point &operator-=(const point &a){ x-=a.x; y-=a.y; }
  point operator+(const point &a)const{ return (point){x+a.x,y+a.y}; }
  point operator-(const point &a)const{ return (point){x-a.x,y-a.y}; }
  operator point<double>()const{ return (point<double>){x,y}; }
};

template<class T>
struct segment{
  point<T> a,b;
  operator segment<double>()const{ return (segment<double>){a,b}; }
};

template<class T>
T cross(const point<T> &a,const point<T> &b){ return a.x*b.y-a.y*b.x; }

enum{CCW=1,CW=-1,ON=0};
template<class T>
int ccw(const point<T> &a,const point<T> &b,const point<T> &c){
  T rdir=cross(b-a,c-a);
  if(rdir>0) return CCW;
  if(rdir<0) return CW;
  return ON;
}

template<class T>
bool intersect(const segment<T> &S1,const segment<T> &S2){
  if(max(S1.a.x,S1.b.x)<min(S2.a.x,S2.b.x)
     || max(S1.a.y,S1.b.y)<min(S2.a.y,S2.b.y)
     || max(S2.a.x,S2.b.x)<min(S1.a.x,S1.b.x)
     || max(S2.a.y,S2.b.y)<min(S1.a.y,S1.b.y)) return false;
  return ccw(S1.a,S1.b,S2.a)*ccw(S1.a,S1.b,S2.b)<=0
    && ccw(S2.a,S2.b,S1.a)*ccw(S2.a,S2.b,S1.b)<=0;
}

int main()
{
  int ax,ay,bx,by;
  cin>>ax>>ay>>bx>>by;
  point<int> a={ax,ay};
  point<int> b={bx,by};
  segment<int> ab={a,b};
  int n;
  cin>>n;
  int cnt=0;
  point<int> p[n];
  for(int i=0;i<n;i++)
    {
      int x,y;
      cin>>x>>y;
      p[i].x=x;
      p[i].y=y;
    }
  for(int i=0;i<n-1;i++)
    {
      segment<int> tmp={p[i],p[i+1]};
	if(intersect(ab,tmp))
	  {
	    cnt++;
	  }
    }
  segment<int> tmp={p[n-1],p[0]};
  if(intersect(ab,tmp))
    {
      cnt++;
    }

  cout<<(cnt/2)+1<<endl;

}