import itertools

def sol():

    n=int(eval(input()))

    d=list(itertools.permutations([i for i in range(n)]))

    w=[]

    for i in range(n):

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

        w.append((x,y))

    ans=chk=0

    for i in d:

        for j in range(len(i)-1):

            ans+=((w[i[j]][0]-w[i[j+1]][0])**2+(w[i[j]][1]-w[i[j+1]][1])**2)**0.5

    print((ans/len(d)))



if __name__=="__main__":

    sol()

import itertools



n=int(eval(input()))

l=[int(i) for i in range(n)]

ans=chk=p=0

w=[]

for i in range(n):

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

    w.append((x,y))

for i in list(itertools.permutations(l,n)):

    chk=0

    for j in range(n-1):

        chk+=((w[i[j]][0]-w[i[j+1]][0])**2+(w[i[j]][1]-w[i[j+1]][1])**2)**0.5

    ans+=chk

    p+=1



print((ans/p))
import itertools


def sol():
    n = int(eval(input()))
    d = list(itertools.permutations([i for i in range(n)]))
    w = []
    for i in range(n):
        x, y = list(map(int, input().split()))
        w.append((x, y))
    ans = chk = 0
    for i in d:
        for j in range(len(i) - 1):
            ans += (
                (w[i[j]][0] - w[i[j + 1]][0]) ** 2 + (w[i[j]][1] - w[i[j + 1]][1]) ** 2
            ) ** 0.5
    print((ans / len(d)))


if __name__ == "__main__":
    sol()

import itertools

n = int(eval(input()))
l = [int(i) for i in range(n)]
ans = chk = p = 0
w = []
for i in range(n):
    x, y = list(map(int, input().split()))
    w.append((x, y))
for i in list(itertools.permutations(l, n)):
    chk = 0
    for j in range(n - 1):
        chk += (
            (w[i[j]][0] - w[i[j + 1]][0]) ** 2 + (w[i[j]][1] - w[i[j + 1]][1]) ** 2
        ) ** 0.5
    ans += chk
    p += 1
print((ans / p))

- -def sol(): -    n = int(eval(input())) -    d = list(itertools.permutations([i for i in range(n)])) -    w = [] -    for i in range(n): -        x, y = list(map(int, input().split())) -        w.append((x, y)) -    ans = chk = 0 -    for i in d: -        for j in range(len(i) - 1): -            ans += ( -                (w[i[j]][0] - w[i[j + 1]][0]) ** 2 + (w[i[j]][1] - w[i[j + 1]][1]) ** 2 -            ) ** 0.5 -    print((ans / len(d))) - - -if __name__ == "__main__": -    sol() +n = int(eval(input())) +l = [int(i) for i in range(n)] +ans = chk = p = 0 +w = [] +for i in range(n): +    x, y = list(map(int, input().split())) +    w.append((x, y)) +for i in list(itertools.permutations(l, n)): +    chk = 0 +    for j in range(n - 1): +        chk += ( +            (w[i[j]][0] - w[i[j + 1]][0]) ** 2 + (w[i[j]][1] - w[i[j + 1]][1]) ** 2 +        ) ** 0.5 +    ans += chk +    p += 1 +print((ans / p))