#include <iostream>
#include <iomanip>
#include <complex>
#include <vector>
#include <algorithm>
#include <cmath>
#include <array>
#include <map>
using namespace std;
const double EPS = 1e-10;
const double INF = 1e12;
#define EQ(n,m) (abs((n)-(m)) < EPS)
#define X real()
#define Y imag()

typedef complex<double> P;
typedef vector<P> VP;
struct L : array<P, 2>{
    L(const P& a, const P& b){ at(0)=a; at(1)=b; }
    L(){}
};

namespace std{
    bool operator < (const P& a, const P& b){
        return !EQ(a.X,b.X) ? a.X<b.X : a.Y+EPS<b.Y;
    }
    bool operator == (const P& a, const P& b){
        return abs(a-b) < EPS;
    }
}

double dot(P a, P b){
    return (conj(a)*b).X;
}
double cross(P a, P b){
    return (conj(a)*b).Y;
}
int ccw(P a, P b, P c){
    b -= a;
    c -= a;
    if(cross(b,c) > EPS) return +1; //ccw
    if(cross(b,c) < -EPS) return -1; //cw
    if(dot(b,c) < -EPS) return +2; //c-a-b
    if(abs(c)-abs(b) > EPS) return -2; //a-b-c
    return 0; //a-c-b
}

bool intersectSP(const L& s, const P &p){
    return abs(cross(s[0]-p, s[1]-p))<EPS  && dot(s[0]-p, s[1]-p)<EPS;
}
bool intersectSS(const L& a, const L& b){
    return ( ccw(a[0],a[1],b[0]) *ccw(a[0],a[1],b[1]) <= 0 ) &&
        ( ccw(b[0],b[1],a[0]) *ccw(b[0],b[1],a[1]) <= 0 );
}

bool isParallel(const P &a, const P &b){
    return abs(cross(a,b)) < EPS;
}
bool isParallel(const L &a, const L &b){
    return isParallel(a[1]-a[0], b[1]-b[0]);
}
P crosspointLL(const L &l, const L &m) {
  double A = cross(l[1]-l[0], m[1]-m[0]);
  double B = cross(l[1]-l[0], l[1]-m[0]);
  return m[0] + B/A *(m[1]-m[0]);
}

double getarea(const VP &poly){
    double ret = 0;
    for (int i=0; i<(int)poly.size(); i++){ 
        ret += cross(poly[i], poly[(i+1)%poly.size()]);
    }
    return ret*0.5;
}
int in_poly(const P &p, const VP &poly){
    int n = poly.size();
    int ret = -1;
    for(int i=0; i<n; i++){
        P a = poly[i]-p;
        P b = poly[(i+1)%n]-p;
        if(a.Y > b.Y) swap(a,b);
        if(intersectSP(L(a,b), P(0,0))) return 0;
        if(a.Y<=0 && b.Y>0 && cross(a,b)<0) ret = -ret;
    }
    return ret;
}

VP convex(VP v){
    VP ret;
    int n = v.size();
    sort(v.begin(), v.end());
    for(int i=0; i<n; i++){
        while((int)ret.size()>1 && cross(ret.back()-ret[ret.size()-2], v[i]-ret.back()) < EPS){
            ret.pop_back();
        }
        ret.push_back(v[i]);
    }
    int t = ret.size();
    for(int i=n-2; i>=0; i--){
        while((int)ret.size()>t && cross(ret.back()-ret[ret.size()-2], v[i]-ret.back()) < EPS){
            ret.pop_back();
        }
        ret.push_back(v[i]);
    }
    if((int)ret.size() > 1) ret.pop_back();
    return ret;
}

VP convex_cut(const VP& p, const L& l){
    VP ret;
    int n = p.size();
    for(int i=0; i<n; i++){
        P curr = p[i];
        P next = p[(i+1)%n];
        if(ccw(l[0], l[1], curr) != -1) ret.push_back(curr);
        if(ccw(l[0], l[1], curr) *ccw(l[0], l[1], next) == -1){
            ret.push_back(crosspointLL(L(curr, next), l));
        }
    }
    return ret;
}

VP convex_intersect(VP a, VP b){
    VP plist;
    int n=a.size(), m=b.size();
    for(int r=0; r<2; r++){
        for(int i=0; i<n; i++){
            if(in_poly(a[i], b) >= 0){
                plist.push_back(a[i]);
            }
        }
        swap(n, m);
        swap(a, b);
    }
    for(int i=0; i<n; i++){
        for(int j=0; j<m; j++){
            L la(a[i], a[(i+1)%n]), lb(b[j], b[(j+1)%m]);
            if(!isParallel(la, lb) && intersectSS(la, lb)){
                plist.push_back(crosspointLL(la, lb));
            }
        }
    }
    return convex(plist);
}

vector<VP> volonoi_division(VP plist, VP poly){
    int m = plist.size();
    vector<VP> ret(m);
    for(int i=0; i<m; i++){
        VP v = poly;
        for(int j=0; j<m; j++){
            if(i==j) continue;
            P mid = (plist[i]+plist[j])/2.0;
            L bisector(mid, mid +(plist[j]-plist[i])*P(0,1));
            VP cut = convex_cut(v, bisector);
            v = convex_intersect(v, cut);
        }
        ret[i] = v;
    }
    return ret;
}

int main(){
    while(1){
        int n;
        cin >> n;
        if(n==0) break;

        VP plist(n);
        for(int i=0; i<n; i++){
            double x,y;
            cin >> x >> y;
            plist[i] = P(x, y);
        }
        VP sq{P(0, 0), P(4, 0), P(4, 4), P(0, 4)};
        vector<VP> poly = volonoi_division(plist, sq);

        vector<L> edge;
        for(int i=0; i<n; i++){
            int m = poly[i].size();
            for(int j=0; j<m; j++){
                edge.push_back(L(poly[i][j], poly[i][(j+1)%m]));
            }
        }
        int m = edge.size();
        map<P, int> conv;
        int count = 0;
        for(int i=0; i<m; i++){
            for(int d=0; d<2; d++){
                if(conv.count(edge[i][d]) == 0){
                    conv[edge[i][d]] = count;
                    count++;
                }
            }
        }

        vector<vector<double> > adj(count, vector<double>(count, INF));
        for(int i=0; i<(int)edge.size(); i++){
            int idx0 = conv[edge[i][0]];
            int idx1 = conv[edge[i][1]];
            if(EQ(edge[i][0].Y, edge[i][1].Y) && (EQ(edge[i][0].Y, 0) || EQ(edge[i][0].Y, 4))){
                continue;
            }
            if(EQ(edge[i][0].X, edge[i][1].X) && (EQ(edge[i][0].X, 0) || EQ(edge[i][0].X, 4))){
                adj[idx0][idx1] = adj[idx1][idx0] = 0;
                continue;
            }
            adj[idx0][idx1] = adj[idx1][idx0] = abs(edge[i][1] -edge[i][0]);
        }
        for(int k=0; k<count; k++){
            for(int i=0; i<count; i++){
                for(int j=0; j<count; j++){
                    adj[i][j] = min(adj[i][j], adj[i][k] +adj[k][j]);
                }
            }
        }

        double ans = adj[conv[P(0,0)]][conv[P(4,4)]];
        if(ans == INF){
            cout << "impossible" << endl;
        }else{
            cout << fixed << setprecision(10);
            cout << ans << endl;
        }
    }
    return 0;
}


Measured memory_bytes: 3320