#include<bits/stdc++.h>
using namespace std;
typedef long double db;

const int MAXN=1005;
const db PI=acos(-1.0);
const db eps=1e-8;

inline int sgn(db x)
{
    if(x>eps)return 1;
    if(x<-eps)return -1;
    return 0;
}

struct Point
{
    db x,y;
    explicit Point(db _x=0,db _y=0):x(_x),y(_y){}
    Point operator + (const Point &t)const { return Point(x+t.x,y+t.y); }
    Point operator - (const Point &t)const { return Point(x-t.x,y-t.y); }
    Point operator * (db t)const { return Point(x*t,y*t); }
    db operator * (const Point &t)const { return x*t.y-y*t.x; }
    db operator ^ (const Point &t)const { return x*t.x+y*t.y; }
    db ang(const Point& t)const {
        db r=atan2((*this)*t,(*this)^t);
        return (r<0 ? r+2*PI : r);
    };
    Point rot(db ang)const { return Point(x*cos(ang)-y*sin(ang),x*sin(ang)+y*cos(ang)); }
    db len()const { return sqrt(x*x+y*y); }
}p[MAXN];

int main()
{
    int n;
    scanf("%d",&n),n++;
    p[0]=Point(-2,0),p[1]=Point(0,0);
    for(int i=2;i<=n;i++)
        scanf("%Lf%Lf",&p[i].x,&p[i].y);
    Point head=p[n]+Point(0,1),tail=p[n],center=p[n];
    bool head_up=1;
    db res=0;
    while(sgn(head.y)>0)
    {
        Point hit=(head_up ? head : tail);
        Point t_center;
        bool t_head_up=0;
        db ang=2*PI,len=(hit-center).len();
        for(int i=0;i<=n;i++)
        {
            if(sgn(p[i].x-center.x)>=0)break;
            if(sgn((p[i]-center).len()-len)>0)continue;
            db tmp=(hit-center).ang(p[i]-center);
            if(tmp<ang)ang=tmp,t_center=p[i],t_head_up=head_up;
        }
        for(int i=0;i<n;i++)
        {
            Point s=p[i],e=(sgn(p[i+1].x-center.x)>0 ? center : p[i+1]);
            if(sgn((s-center).len()-len)<0 || sgn((e-center).len()-len)>0)continue;
            db l=0,r=1;
            for(int _=0;_<50;_++)
            {
                db m=(l+r)/2;
                Point t=(s-e)*m+e;
                if((t-center).len()>len)r=m;
                else l=m;
            }
            Point t_hit=(s-e)*((l+r)/2)+e;
            db tmp=(hit-center).ang(t_hit-center);
            if(tmp<ang)ang=tmp,t_center=t_hit,t_head_up=!head_up;
            break;
        }
        res+=(head-center).len()*ang;
        head=center+(head-center).rot(ang);
        tail=center+(tail-center).rot(ang);
        center=t_center;
        head_up=t_head_up;
    }
    return 0*printf("%.12Lf\n",res);
}