import math

import itertools

a=[]

ans=0

n=int(eval(input()))

m=1

for i in range(1,n+1):

  m*=i

for i in range(n):

    x,y=list(map(int,input().split()))

    a.append([x,y])

for t in itertools.permutations(list(range(n)),n):

  for i in range(n-1):

    ans+=math.sqrt((a[t[i]][0]-a[t[i+1]][0])**2+(a[t[i]][1]-a[t[i+1]][1])**2)

ans=ans/m

print(ans)
def resolve():

    n=int(eval(input()))

    x = [0] * n

    y = [0] * n

    ans=0

    t=1

    for i in range(1,n):

        t*=i

    for i in range(n):

        x[i], y[i] = list(map(int, input().split()))

    for i in range(n):

        for j in range(i+1,n):

            ans+=((x[i]-x[j])**2+(y[i]-y[j])**2)**0.5

    print((2*ans/n))

resolve()
import math
import itertools

a = []
ans = 0
n = int(eval(input()))
m = 1
for i in range(1, n + 1):
    m *= i
for i in range(n):
    x, y = list(map(int, input().split()))
    a.append([x, y])
for t in itertools.permutations(list(range(n)), n):
    for i in range(n - 1):
        ans += math.sqrt(
            (a[t[i]][0] - a[t[i + 1]][0]) ** 2 + (a[t[i]][1] - a[t[i + 1]][1]) ** 2
        )
ans = ans / m
print(ans)

def resolve():
    n = int(eval(input()))
    x = [0] * n
    y = [0] * n
    ans = 0
    t = 1
    for i in range(1, n):
        t *= i
    for i in range(n):
        x[i], y[i] = list(map(int, input().split()))
    for i in range(n):
        for j in range(i + 1, n):
            ans += ((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) ** 0.5
    print((2 * ans / n))


resolve()

-import math -import itertools +def resolve(): +    n = int(eval(input())) +    x = [0] * n +    y = [0] * n +    ans = 0 +    t = 1 +    for i in range(1, n): +        t *= i +    for i in range(n): +        x[i], y[i] = list(map(int, input().split())) +    for i in range(n): +        for j in range(i + 1, n): +            ans += ((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) ** 0.5 +    print((2 * ans / n)) -a = [] -ans = 0 -n = int(eval(input())) -m = 1 -for i in range(1, n + 1): -    m *= i -for i in range(n): -    x, y = list(map(int, input().split())) -    a.append([x, y]) -for t in itertools.permutations(list(range(n)), n): -    for i in range(n - 1): -        ans += math.sqrt( -            (a[t[i]][0] - a[t[i + 1]][0]) ** 2 + (a[t[i]][1] - a[t[i + 1]][1]) ** 2 -        ) -ans = ans / m -print(ans) + +resolve()