#include<cstdio>

#include<cmath>

#include<algorithm>

using namespace std;

#define EPS 0.00000001

struct Point

{

	double x,y;

	Point(){}

	Point(const double &X,const double &Y){x=X;y=Y;}

	double Length()

	  {

	  	return sqrt(x*x+y*y);

	  }

}p[3],a[31];

struct data

{

	Point p;

	int I,J;

	data(){}

	data(const Point &P,const int &II,const int &JJ)

	  {

	  	p=P;

	  	I=II;

	  	J=JJ;

	  }

}b[31];

int e;

typedef Point Vector;

Vector operator - (const Point &a,const Point &b)

{

    return Vector(a.x-b.x,a.y-b.y);

}

Vector operator + (const Vector &a,const 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;

}

Vector operator * (const double &K,const Vector &v)

{

    return Vector(K*v.x,K*v.y);

}

Point GetIntersection(Point P,Vector v,Point Q,Vector w)

{

    return P+(Cross(w,P-Q)/Cross(v,w))*v;

}

double dot(Vector a,Vector b)

{

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

}

bool InLine(Point a,Point P,Point Q)

{

	if(fabs(Cross(a-P,Q-P))>=EPS)

	  return 0;

	return dot(a-P,a-Q)<EPS;

}

int n;

int main()

{

//	freopen("s.in","r",stdin);

	double sum=0;

	for(int i=0;i<2;++i)

	  scanf("%lf%lf",&p[i].x,&p[i].y);

	scanf("%d",&n);

	for(int i=0;i<n;++i)

	  scanf("%lf%lf",&a[i].x,&a[i].y);

	for(int i=0;i<n;++i)

	  if(fabs(Cross(p[1]-a[(i+1)%n],a[i]-p[0]))<EPS)

	    {

	      printf("%.9lf\n",(p[1]-p[0]).Length());

	      return 0;

	    }

	for(int i=0;i<n;++i)

	  sum+=(a[(i+1)%n]-a[i]).Length();

					/*double sum2=0;

	  	  	  	  	for(int k=i+1;k<j;++k)

	  	  	  	  	  sum2+=(a[(k+1)%n]-a[k]).Length();

	  	  	  	  	sum2+=(a[i+1]-tp).Length();

	  	  	  	  	sum2+=(tp2-a[j]).Length();

	  	  	  	  	double td=min((tp-p[0]).Length()+(tp2-p[1]).Length(),

							(tp2-p[0]).Length()+(tp-p[1]).Length());

	  	  	  	  	printf("%.9lf\n",td+min(sum-sum2,min(sum2,(tp2-tp).Length()*2.0)));*/

	for(int i=0;i<n;++i)

	  {

	  	Point tp=GetIntersection(p[1],p[1]-p[0],a[i],a[(i+1)%n]-a[i]);

	  	if(InLine(tp,a[i],a[(i+1)%n]) && InLine(tp,p[0],p[1]))

	  	  {

	  	  	for(int j=i+1;j<n;++j)

	  	  	  {

	  	  	  	Point tp2=GetIntersection(p[1],p[1]-p[0],a[j],a[(j+1)%n]-a[j]);

	  	  	  	if(InLine(tp2,a[j],a[(j+1)%n]) && InLine(tp2,p[0],p[1]))

	  	  	  	  b[++e]=data(tp2,i,j);

	  	  	  }

	  	  	for(int j=1;j<=e;++j)

	  	  	  if(fabs(b[j].p.x-tp.x)>=EPS || fabs(b[j].p.y-tp.y)>=EPS)

	  	  	    {

	  	  	      int I=b[j].I,J=b[j].J;

	  	  	      Point tp2=b[j].p;

	  	  	      double sum2=0;

	  	  	  	  for(int k=I+1;k<J;++k)

	  	  	  	  	sum2+=(a[(k+1)%n]-a[k]).Length();

	  	  	  	  sum2+=(a[I+1]-tp).Length();

	  	  	  	  sum2+=(tp2-a[J]).Length();

	  	  	  	  double td=min((tp-p[0]).Length()+(tp2-p[1]).Length(),

					(tp2-p[0]).Length()+(tp-p[1]).Length());

	  	  	  	  printf("%.9lf\n",td+min(sum-sum2,min(sum2,(tp2-tp).Length()*2.0)));

	  	  	  	  return 0;

	  	  	    }

	  	  		  int I=b[1].I,J=b[1].J;

	  	  	      Point tp2=b[1].p;

	  	  	      double sum2=0;

	  	  	  	  for(int k=I+1;k<J;++k)

	  	  	  	  	sum2+=(a[(k+1)%n]-a[k]).Length();

	  	  	  	  sum2+=(a[I+1]-tp).Length();

	  	  	  	  sum2+=(tp2-a[J]).Length();

	  	  	  	  double td=min((tp-p[0]).Length()+(tp2-p[1]).Length(),

					(tp2-p[0]).Length()+(tp-p[1]).Length());

	  	  	  	  printf("%.9lf\n",td+min(sum-sum2,min(sum2,(tp2-tp).Length()*2.0)));

	  	  	  	  return 0;

	  	  }

	  }

	printf("%.9lf\n",(p[1]-p[0]).Length());

	return 0;

}