import math

import itertools



N=int(eval(input()))

X=list()

Y=list()

for i in range(N):

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

   X.append(x)

   Y.append(y)



numbers=[c for c in range(N)]

kyori=0

for i in itertools.permutations(numbers,N):

   for j in range(N-1):

      mae=i[j]

      usiro=i[j+1]

      kyori+=(((X[usiro]-X[mae])**2)+((Y[usiro]-Y[mae])**2))**0.5



N_f=math.factorial(N)

print((kyori/N_f))
from sys import stdin

def main():

    #入力

    readline=stdin.readline

    N=int(readline())

    N_kaijo=1

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

        N_kaijo*=i



    x=[]

    y=[]

    for i in range(N):

        x_i,y_i=list(map(int,readline().split()))

        x.append(x_i)

        y.append(y_i)

    

    res=0

    for i in range(N-1):

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

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

            res+=l*2/N



    print(res)

if __name__=="__main__":

    main()
import math
import itertools

N = int(eval(input()))
X = list()
Y = list()
for i in range(N):
    x, y = list(map(int, input().split()))
    X.append(x)
    Y.append(y)
numbers = [c for c in range(N)]
kyori = 0
for i in itertools.permutations(numbers, N):
    for j in range(N - 1):
        mae = i[j]
        usiro = i[j + 1]
        kyori += (((X[usiro] - X[mae]) ** 2) + ((Y[usiro] - Y[mae]) ** 2)) ** 0.5
N_f = math.factorial(N)
print((kyori / N_f))

from sys import stdin


def main():
    # 入力
    readline = stdin.readline
    N = int(readline())
    N_kaijo = 1
    for i in range(1, N + 1):
        N_kaijo *= i
    x = []
    y = []
    for i in range(N):
        x_i, y_i = list(map(int, readline().split()))
        x.append(x_i)
        y.append(y_i)
    res = 0
    for i in range(N - 1):
        for j in range(i + 1, N):
            l = ((x[j] - x[i]) ** 2 + (y[j] - y[i]) ** 2) ** 0.5
            res += l * 2 / N
    print(res)


if __name__ == "__main__":
    main()

-import math -import itertools +from sys import stdin -N = int(eval(input())) -X = list() -Y = list() -for i in range(N): -    x, y = list(map(int, input().split())) -    X.append(x) -    Y.append(y) -numbers = [c for c in range(N)] -kyori = 0 -for i in itertools.permutations(numbers, N): -    for j in range(N - 1): -        mae = i[j] -        usiro = i[j + 1] -        kyori += (((X[usiro] - X[mae]) ** 2) + ((Y[usiro] - Y[mae]) ** 2)) ** 0.5 -N_f = math.factorial(N) -print((kyori / N_f)) + +def main(): +    # 入力 +    readline = stdin.readline +    N = int(readline()) +    N_kaijo = 1 +    for i in range(1, N + 1): +        N_kaijo *= i +    x = [] +    y = [] +    for i in range(N): +        x_i, y_i = list(map(int, readline().split())) +        x.append(x_i) +        y.append(y_i) +    res = 0 +    for i in range(N - 1): +        for j in range(i + 1, N): +            l = ((x[j] - x[i]) ** 2 + (y[j] - y[i]) ** 2) ** 0.5 +            res += l * 2 / N +    print(res) + + +if __name__ == "__main__": +    main()