def main():

  import sys

  input = sys.stdin.readline

  import math 

  import itertools 

  n=int(eval(input()))

  xy=[list(map(int,input().split())) for _ in [0]*n]

  l=''.join([str(i) for i in range(n)])

  sum=0

  for i in itertools.permutations(l):

    i=''.join(list(i))

    for j in range(n-1):

      a,b=int(i[j+1]),int(i[j])

      sum+=pow(pow(xy[a][0]-xy[b][0],2)+pow(xy[a][1]-xy[b][1],2),0.5)

  m=math.factorial(n)

  print((sum/m))

if __name__ == '__main__':

    main()
def main():

  import sys

  input = sys.stdin.readline

  n=int(eval(input()))

  xy=[list(map(int,input().split())) for _ in [0]*n]

  ans=0

  for i in range(n):

    xi,yi=xy[i][0],xy[i][1]

    for j in range(n):

      xj,yj=xy[j][0],xy[j][1]

      ans+=pow(pow(xi-xj,2)+pow(yi-yj,2),0.5)

  print((ans/n))

if __name__ == '__main__':

    main()
def main():
    import sys

    input = sys.stdin.readline
    import math
    import itertools

    n = int(eval(input()))
    xy = [list(map(int, input().split())) for _ in [0] * n]
    l = "".join([str(i) for i in range(n)])
    sum = 0
    for i in itertools.permutations(l):
        i = "".join(list(i))
        for j in range(n - 1):
            a, b = int(i[j + 1]), int(i[j])
            sum += pow(pow(xy[a][0] - xy[b][0], 2) + pow(xy[a][1] - xy[b][1], 2), 0.5)
    m = math.factorial(n)
    print((sum / m))


if __name__ == "__main__":
    main()

def main():
    import sys

    input = sys.stdin.readline
    n = int(eval(input()))
    xy = [list(map(int, input().split())) for _ in [0] * n]
    ans = 0
    for i in range(n):
        xi, yi = xy[i][0], xy[i][1]
        for j in range(n):
            xj, yj = xy[j][0], xy[j][1]
            ans += pow(pow(xi - xj, 2) + pow(yi - yj, 2), 0.5)
    print((ans / n))


if __name__ == "__main__":
    main()

-    import math -    import itertools - -    l = "".join([str(i) for i in range(n)]) -    sum = 0 -    for i in itertools.permutations(l): -        i = "".join(list(i)) -        for j in range(n - 1): -            a, b = int(i[j + 1]), int(i[j]) -            sum += pow(pow(xy[a][0] - xy[b][0], 2) + pow(xy[a][1] - xy[b][1], 2), 0.5) -    m = math.factorial(n) -    print((sum / m)) +    ans = 0 +    for i in range(n): +        xi, yi = xy[i][0], xy[i][1] +        for j in range(n): +            xj, yj = xy[j][0], xy[j][1] +            ans += pow(pow(xi - xj, 2) + pow(yi - yj, 2), 0.5) +    print((ans / n))