N = int(eval(input()))

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

ans = 0

c = 0



def dfs(i, dis, passed):

    if len(passed) == N:

        global c, ans

        c += 1

        ans += dis

        return

    x = P[i][0]

    y = P[i][1]

    for ii in range(N):

        if P[ii] in passed:

            continue

        xx = P[ii][0]

        yy = P[ii][1]

        passed.append([xx, yy])

        dfs(ii, dis + ((x - xx)**2 + (y - yy)**2)**0.5, passed)

        passed.pop(-1)



for i in range(N):

    dfs(i, 0, [P[i]])

print((ans / c))
import math



N = int(eval(input()))

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

dis = 0

c = 0



for i in range(N):

    xi = P[i][0]

    yi = P[i][1]

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

        xj = P[ii][0]

        yj = P[ii][1]

        dis += ((xi - xj)**2 + (yi - yj)**2)**0.5

        c += 1

print((dis / c * (N - 1)))

        

    

    
N = int(eval(input()))
P = [list(map(int, input().split())) for _ in range(N)]
ans = 0
c = 0


def dfs(i, dis, passed):
    if len(passed) == N:
        global c, ans
        c += 1
        ans += dis
        return
    x = P[i][0]
    y = P[i][1]
    for ii in range(N):
        if P[ii] in passed:
            continue
        xx = P[ii][0]
        yy = P[ii][1]
        passed.append([xx, yy])
        dfs(ii, dis + ((x - xx) ** 2 + (y - yy) ** 2) ** 0.5, passed)
        passed.pop(-1)


for i in range(N):
    dfs(i, 0, [P[i]])
print((ans / c))

import math

N = int(eval(input()))
P = [list(map(int, input().split())) for _ in range(N)]
dis = 0
c = 0
for i in range(N):
    xi = P[i][0]
    yi = P[i][1]
    for ii in range(i + 1, N):
        xj = P[ii][0]
        yj = P[ii][1]
        dis += ((xi - xj) ** 2 + (yi - yj) ** 2) ** 0.5
        c += 1
print((dis / c * (N - 1)))

+import math + -ans = 0 +dis = 0 - - -def dfs(i, dis, passed): -    if len(passed) == N: -        global c, ans +for i in range(N): +    xi = P[i][0] +    yi = P[i][1] +    for ii in range(i + 1, N): +        xj = P[ii][0] +        yj = P[ii][1] +        dis += ((xi - xj) ** 2 + (yi - yj) ** 2) ** 0.5 -        ans += dis -        return -    x = P[i][0] -    y = P[i][1] -    for ii in range(N): -        if P[ii] in passed: -            continue -        xx = P[ii][0] -        yy = P[ii][1] -        passed.append([xx, yy]) -        dfs(ii, dis + ((x - xx) ** 2 + (y - yy) ** 2) ** 0.5, passed) -        passed.pop(-1) - - -for i in range(N): -    dfs(i, 0, [P[i]]) -print((ans / c)) +print((dis / c * (N - 1)))