import itertools



N=int(eval(input()))

x=[0]*N

y=[0]*N

for i in range(N):

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



l=[i for i in range(N)]

dis=0

cnt=0

for v in itertools.permutations(l):

  #print(v)

  cnt+=1

  for i in range(1,N):

    dis+=((x[v[i]]-x[v[i-1]])**2+(y[v[i]]-y[v[i-1]])**2)**0.5



print((dis/cnt))
import sys

input = sys.stdin.buffer.readline



N = int(eval(input()))

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



def dist(xy1, xy2):

  x1, y1 = xy1

  x2, y2 = xy2

  return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5



tmp = 0

for i in range(N):

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

    tmp += dist(XY[i], XY[j]) / (N / 2)



print(tmp)
import itertools

N = int(eval(input()))
x = [0] * N
y = [0] * N
for i in range(N):
    x[i], y[i] = list(map(int, input().split()))
l = [i for i in range(N)]
dis = 0
cnt = 0
for v in itertools.permutations(l):
    # print(v)
    cnt += 1
    for i in range(1, N):
        dis += ((x[v[i]] - x[v[i - 1]]) ** 2 + (y[v[i]] - y[v[i - 1]]) ** 2) ** 0.5
print((dis / cnt))

import sys

input = sys.stdin.buffer.readline
N = int(eval(input()))
XY = [list(map(int, input().split())) for _ in range(N)]


def dist(xy1, xy2):
    x1, y1 = xy1
    x2, y2 = xy2
    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5


tmp = 0
for i in range(N):
    for j in range(i + 1, N):
        tmp += dist(XY[i], XY[j]) / (N / 2)
print(tmp)

-import itertools +import sys +input = sys.stdin.buffer.readline -x = [0] * N -y = [0] * N +XY = [list(map(int, input().split())) for _ in range(N)] + + +def dist(xy1, xy2): +    x1, y1 = xy1 +    x2, y2 = xy2 +    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5 + + +tmp = 0 -    x[i], y[i] = list(map(int, input().split())) -l = [i for i in range(N)] -dis = 0 -cnt = 0 -for v in itertools.permutations(l): -    # print(v) -    cnt += 1 -    for i in range(1, N): -        dis += ((x[v[i]] - x[v[i - 1]]) ** 2 + (y[v[i]] - y[v[i - 1]]) ** 2) ** 0.5 -print((dis / cnt)) +    for j in range(i + 1, N): +        tmp += dist(XY[i], XY[j]) / (N / 2) +print(tmp)