import itertools

import math

def dis(X,Y):

    return ((X[0]-Y[0])**2+(X[1]-Y[1])**2)**.5

def main():

    n=int(eval(input()))

    L=[list(map(int,input().split())) for _ in range(n)]

    S=itertools.permutations([i for i in range(1,n+1)])

    ans=0

    for s in S:

        for i in range(n-1):

            a = s[i] - 1

            b = s[i+1] - 1

            ans += dis(L[a],L[b])

    ans /= math.factorial(n)

    print(ans)

main()
n = int(eval(input()))

L = [list(map(int, input().split())) for _ in range(n)]

ans = 0

# (edgesの長さの総和)/(edgeの本数)*(1つのpathを構成するedgesの数)

# = (edgesの長さの総和)/ (n*(n-1)) * (n-1)

for x1, y1 in L:

    for x2, y2 in L:

        ans += ((x1 - x2)**2 + (y1 - y2)**2)**.5

print((ans/n))
import itertools
import math


def dis(X, Y):
    return ((X[0] - Y[0]) ** 2 + (X[1] - Y[1]) ** 2) ** 0.5


def main():
    n = int(eval(input()))
    L = [list(map(int, input().split())) for _ in range(n)]
    S = itertools.permutations([i for i in range(1, n + 1)])
    ans = 0
    for s in S:
        for i in range(n - 1):
            a = s[i] - 1
            b = s[i + 1] - 1
            ans += dis(L[a], L[b])
    ans /= math.factorial(n)
    print(ans)


main()

n = int(eval(input()))
L = [list(map(int, input().split())) for _ in range(n)]
ans = 0
# (edgesの長さの総和)/(edgeの本数)*(1つのpathを構成するedgesの数)
# = (edgesの長さの総和)/ (n*(n-1)) * (n-1)
for x1, y1 in L:
    for x2, y2 in L:
        ans += ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5
print((ans / n))

-import itertools -import math - - -def dis(X, Y): -    return ((X[0] - Y[0]) ** 2 + (X[1] - Y[1]) ** 2) ** 0.5 - - -def main(): -    n = int(eval(input())) -    L = [list(map(int, input().split())) for _ in range(n)] -    S = itertools.permutations([i for i in range(1, n + 1)]) -    ans = 0 -    for s in S: -        for i in range(n - 1): -            a = s[i] - 1 -            b = s[i + 1] - 1 -            ans += dis(L[a], L[b]) -    ans /= math.factorial(n) -    print(ans) - - -main() +n = int(eval(input())) +L = [list(map(int, input().split())) for _ in range(n)] +ans = 0 +# (edgesの長さの総和)/(edgeの本数)*(1つのpathを構成するedgesの数) +# = (edgesの長さの総和)/ (n*(n-1)) * (n-1) +for x1, y1 in L: +    for x2, y2 in L: +        ans += ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5 +print((ans / n))