//Andrew算法//核心思想：（以下凸包为例）顺向，左边就拐，否则后撤，直到能左拐。 #include<cstdio>#include<cmath>#include<algorithm>using namespace std;const int N=1e5+5;const double eps=1e-10;struct Vector{    double x,y;    Vector(double x=0,double y=0):x(x),y(y){}}p[N],ch[N];int n;Vector operator + (Vector A,Vector B){return Vector(A.x+B.x,A.y+B.y);}//向量加法Vector operator - (Vector A,Vector B){return Vector(A.x-B.x,A.y-B.y);}//向量减法Vector operator * (Vector A,double p){return Vector(A.x*p,A.y*p);}//向量乘以标量Vector operator / (Vector A,double p){return Vector(A.x/p,A.y/p);}//向量除以标量bool operator < (const Vector &a,const Vector &b){//点的坐标排序    return a.x==b.x?a.y<b.y:a.x<b.x;}int dcmp(double x){//三态函数，减少精度问题    if(fabs(x)<eps) return 0;    else return x<0?-1:1;//忘写return WA*1 }bool operator ==(const Vector &a,const Vector &b){//判断同一个点    return dcmp(a.x-b.x)==0&&dcmp(a.y-b.y)==0;}double Dot(Vector A,Vector B){return A.x*B.x+A.y*B.y;}//向量点积double Length(Vector A){return sqrt(Dot(A,A));} //向量长度，点积double Angle(Vector A,Vector B){return acos(Dot(A,B)/Length(A)/Length(B));}//向量转角，逆时针，点积double Cross(Vector A,Vector B){return A.x*B.y-A.y*B.x;}//向量叉积double Area2(Vector A,Vector B,Vector C){return Cross(B-A,C-A);}//三角形有向面积的两倍 //计算凸包，输入点数组p,不重复个数cnt,输出点数组ch。函数返回凸包顶点个数。//如果不希望在凸包的边上有输入点，把两个 <=改成 < int ConvexHull(){    sort(p,p+n);    int cnt=unique(p,p+n)-p;    int m=0;    for(int i=0;i<cnt;i++){        while(m>1&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;        ch[m++]=p[i];    }    int k=m;    for(int i=cnt-2;i>=0;i--){        while(m>k&&Cross(ch[m-1]-ch[m-2],p[i]-ch[m-2])<=0) m--;        ch[m++]=p[i];    }    if(cnt>1) m--;    return m;}int main(){    scanf("%d",&n);    for(int i=0;i<n;i++) scanf("%lf%lf",&p[i].x,&p[i].y);    int c=ConvexHull();double ans=0;    for(int i=0;i<c;i++) ans+=Length(ch[i]-ch[i+1]);    printf("%.1lf\n",ans);    return 0;}/*ConvexHull()段代码来自小白书P272 */