import math
import copy
from copy import deepcopy
import sys
import fractions
# import numpy as np
from functools import reduce
# import statistics
import heapq
import collections
import itertools
sys.setrecursionlimit(100001)

# input = sys.stdin.readline
# sys.setrecursionlimit(10**6)

# ===FUNCTION===

def getInputInt():
    inputNum = int(input())
    return inputNum


def getInputListInt():
    outputData = []
    inputData = input().split()
    outputData = [int(n) for n in inputData]

    return outputData


def getSomeInputInt(n):
    outputDataList = []
    for i in range(n):
        inputData = int(input())
        outputDataList.append(inputData)

    return outputDataList


def getSomeInputListInt(n):
    inputDataList = []
    outputDataList = []
    for i in range(n):
        inputData = input().split()
        inputDataList = [int(n) for n in inputData]
        outputDataList.append(inputDataList)

    return outputDataList


# ===CODE===

n = int(input())


inputDataList = []
data = []
for i in range(n):
    inputData = input().split()
    inputDataList = [int(n) for n in inputData]
    data.append(inputDataList)

mat = [[-1 for i in range(n)] for j in range(n)] #二次元の静的確保


def calc_dis(x1, x2, y1, y2):
    dis = pow(x1 - x2, 2) + pow(y1 - y2, 2)
    dis = math.sqrt(dis)


def path_dis(path):
    dis = 0
    for i in range(len(path)-1):
        dis += mat[path[i]][path[i+1]]
    return dis

for i in range(n):
    for j in range(n):
        mat[i][j] = calc_dis(data[i][0], data[i][1], data[j][0], data[j][1])

perm = list(itertools.permutations(n))

ans = 0
for p in perm:
    tmp_ans = path_dis(p)
    ans += tmp_ans

ans = ans/n
print(ans)





### FIXED CODE ###

import math
import copy
from copy import deepcopy
import sys
import fractions
# import numpy as np
from functools import reduce
# import statistics
import heapq
import collections
import itertools
sys.setrecursionlimit(100001)

# input = sys.stdin.readline
# sys.setrecursionlimit(10**6)

# ===FUNCTION===

def getInputInt():
    inputNum = int(input())
    return inputNum


def getInputListInt():
    outputData = []
    inputData = input().split()
    outputData = [int(n) for n in inputData]

    return outputData


def getSomeInputInt(n):
    outputDataList = []
    for i in range(n):
        inputData = int(input())
        outputDataList.append(inputData)

    return outputDataList


def getSomeInputListInt(n):
    inputDataList = []
    outputDataList = []
    for i in range(n):
        inputData = input().split()
        inputDataList = [int(n) for n in inputData]
        outputDataList.append(inputDataList)

    return outputDataList


# ===CODE===

n = int(input())


inputDataList = []
data = []
for i in range(n):
    inputData = input().split()
    inputDataList = [int(n) for n in inputData]
    data.append(inputDataList)

mat = [[-1 for i in range(n)] for j in range(n)] #二次元の静的確保


def calc_dis(x1, y1, x2, y2):
    dis = pow(x1 - x2, 2) + pow(y1 - y2, 2)
    dis = math.sqrt(dis)
    return dis


def path_dis(path):
    dis = 0
    for i in range(len(path)-1):
        dis += mat[path[i]][path[i+1]]
    return dis

for i in range(n):
    for j in range(n):
        mat[i][j] = calc_dis(data[i][0], data[i][1], data[j][0], data[j][1])

perm = list(itertools.permutations(range(n)))

ans = 0
for p in perm:
    tmp_ans = path_dis(p)
    ans += tmp_ans

ans = ans/len(perm)
print(ans)