#include <bits/stdc++.h>
using namespace std;

struct vec2{
  int64_t x,y;

  vec2 operator -(vec2 const &a) const{
    return {x-a.x,y-a.y};
  }

  long double length() const{
    return hypotl(x,y);
  }
};

int64_t cross(vec2 const &a,vec2 const &b){
  return a.y*b.x-a.x*b.y;
}

bool between(pair<vec2,vec2> const &a,pair<vec2,vec2> const &b){
  int64_t c1=cross(a.second-a.first,b.first-a.first);
  int64_t c2=cross(a.second-a.first,b.second-a.first);
  return (c1<=0 and c2>=0) or (c2<=0 and c1>=0);
}

bool intersects(pair<vec2,vec2> const &a,pair<vec2,vec2> const &b){
  return between(a,b) and between(b,a);
}

int main(){
  int n; cin>>n;
  vector<vec2> v(n*2+4);
  cin>>v[0].x>>v[0].y; v[1]=v[0];
  for (auto it=v.begin()+2; it!=v.begin()+n*2+2; ++it) cin>>it->x>>it->y;
  cin>>v[n*2+2].x>>v[n*2+2].y; v[n*2+3]=v[n*2+2];

  vector<long double> dist(n*2+4,1e80L);
  dist[1]=0;

  for (int i=2; i<n*2+3; i++){
    for (int j=1; j/2<i/2; j++){
      vec2 const &a=v[j];
      vec2 const &b=v[i];
      long double score=dist[j]+(b-a).length();
      if (score>=dist[i]){
        continue;
      }
      bool ok=true;
      for (int k=(j&~0x1)+2; k/2<i/2; k+=2){
        if (not intersects({a,b},{v[k],v[k+1]})){
          ok=false;
          break;
        }
      }
      if (ok){
        dist[i]=score;
      }
    }
  }

  cout.precision(8);
  cout<<fixed<<dist[n*2+2]<<endl;
}