#include <cstdio>

 #include <cmath>

 #include <algorithm>

 using namespace std;

 const int maxn =  + ;

 const double PI = acos(-1.0);

 struct Point

 {

     int x, y;

 }p[maxn];

 double ang[maxn * ];

 typedef Point Vector;

 double inline C2(int n) { return n * (n-) / ; }

 double inline C3(int n) { return n * (n-) /  * (n-) / ; }

 double solve(int n)

 {

     double s = C3(n-), ans = ;

     for(int i = ; i < n; i++)

     {//枚举中心点

         int k = ;

         for(int j = ; j < n; j++) if(j != i)

         {

             ang[k] = atan2(p[j].x-p[i].x, p[j].y-p[i].y);

             ang[k + n - ] = ang[k] + 2.0 * PI;

             k++;

         }

         k = *(n-);

         sort(ang, ang + k);

         int L = , R = ;

         double t = ;

         for(L = ; L < n-; L++)

         {

             double temp = ang[L] + PI;

             while(temp > ang[R]) R++;

             t += C2(R - L - );

         }

         ans += s - t;

     }

     return ans;

 }

 int main()

 {

     //freopen("in.txt", "r", stdin);

     int n, kase = ;

     while(scanf("%d", &n) ==  && n)

     {

         for(int i = ; i < n; i++)

             scanf("%d%d", &p[i].x, &p[i].y);

         double ans = solve(n) / C3(n);

         printf("City %d: %.2f\n", ++kase, ans);

     }

     return ;

 }