text: /* ACM 412 Pi
   * mythnc
   * 2011/11/24 10:50:36   
   * run time: 0.124
   */
  #include <stdio.h>
  #include <math.h>
  
  #define MAXN 49
  
  void input(int *, int);
  int comfactor(int *, int);
  int gcd(int, int);
  
  int main(void)
  {
      int n, pair;
      int list[MAXN];
  
      while (scanf("%d", &n) == 1 && n != 0) {
          input(list, n);
          pair = comfactor(list, n);
          if (pair == 0) {
              printf("No estimate for this data set.\n");
              continue;
          }
          printf("%.6f\n", sqrt(3.0 / pair * n * (n - 1)));
      }
  
      return 0;
  }
  
  /* input: receive input data */
  void input(int *list, int n)
  {
      int i;
  
      for (i = 0; i < n; i++)
          scanf("%d", list + i);
  }
  
  /* comfactor: return no common factors pair numbers */
  int comfactor(int *list, int n)
  {
      int i, j, count;
  
      for (count = i = 0; i < n - 1; i++)
          for (j = i + 1; j < n; j++)
              if (gcd(list[i], list[j]) == 1)
                  count++;
  
      return count;
  }
  
  /* gcd: calculate gcd of a and b */
  int gcd(int a, int b)
  {
      while ((a %= b) && (b %= a))
          ;
  
      return a + b;
  }
  
  