1 #include<cmath> 2 #include<cstdio> 3 #include<algorithm> 4 using namespace std; 5  6 #define MAXN 100+10 7  8 struct Point{ 9     double x, y;10 11     Point(double x = 0, double y = 0): x(x), y(y){}12 13     void scan(){14         scanf("%lf %lf", &x, &y);15     }16 };17 18 typedef Point Vector;19 20 Vector operator - (Vector A, Vector B){21     return Vector(A.x - B.x, A.y - B.y);22 }23 24 double dot(Vector A, Vector B){//点乘25     return A.x * B.x + A.y * B.y;26 }27 double length(Vector A){//向量模28     return sqrt(dot(A, A));29 }30 double cross(Vector A, Vector B){//叉乘31     return A.x * B.y - A.y * B.x;32 }33 34 int n;35 Point p[MAXN], stack[MAXN];36 37 bool input(){38     scanf("%d", &n);39     if(n == 0) return false;40     for(int i = 0; i < n; ++i){41         p[i].scan();42     }43     return true;44 }45 46 bool cmp(Point A, Point B){47     return A.x < B.x || (A.x == B.x && A.y < B.y);48 }49 50 int convex_hull(){//Adrew算法 求凸包51     sort(p, p + n, cmp);52 53     int m = 0;54     for(int i = 0; i < n; ++i){55         while(m > 1 && cross(stack[m - 1] - stack[m - 2], p[i] - stack[m - 2]) <= 0) --m;56         stack[m ++] = p[i];57     }58     int k = m;59     for(int i = n - 1; i >= 0; --i){60         while(m > k && cross(stack[m - 1] - stack[m - 2], p[i] - stack[m - 2]) <= 0) --m;61         stack[m ++] = p[i];62     }63     if(n > 1) --m;64     return m;65 }66 67 void solve(){68     n = convex_hull();69     if(n == 1){//当只有一个点时，绳子长度为070         puts("0.00");71         return ;72     }73     if(n == 2){//当只有两个点时，绳子长度为两点之间的长度74         printf("%.2lf\n", length(stack[0] - stack[1]));75         return ;76     }77     double ans = 0;//绳子的累加长度78     stack[n] = stack[0];//为了算第n点到第1点的距离79     for(int i = 0; i < n; ++i){80         ans += length(stack[i] - stack[i + 1]);81     }82     printf("%.2lf\n", ans);83 }84 85 int main(){86     while(input()){87         solve();88     }89     return 0;90 }