# -*- coding: utf-8 -*-



import sys

from itertools import permutations

from math import hypot



def input(): return sys.stdin.readline().strip()

def list2d(a, b, c): return [[c] * b for i in range(a)]

def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]

def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]

def ceil(x, y=1): return int(-(-x // y))

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]

def Yes(): print('Yes')

def No(): print('No')

def YES(): print('YES')

def NO(): print('NO')

sys.setrecursionlimit(10 ** 9)

INF = 10 ** 18

MOD = 10 ** 9 + 7



N = INT()

XY = []

for i in range(N):

    x, y = MAP()

    XY.append((x, y))



sm = cnt = 0

for perm in permutations(XY):

    dist = 0

    prev = (0, 0)

    for i, (x, y) in enumerate(perm):

        if i == 0:

            prev = (x, y)

        else:

            x2, y2 = prev

            dist += hypot(x-x2, y-y2)

    sm += dist

    cnt += 1

print((sm/cnt))

# -*- coding: utf-8 -*-



import sys

from math import hypot



def input(): return sys.stdin.readline().strip()

def list2d(a, b, c): return [[c] * b for i in range(a)]

def list3d(a, b, c, d): return [[[d] * c for j in range(b)] for i in range(a)]

def list4d(a, b, c, d, e): return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]

def ceil(x, y=1): return int(-(-x // y))

def INT(): return int(eval(input()))

def MAP(): return list(map(int, input().split()))

def LIST(N=None): return list(MAP()) if N is None else [INT() for i in range(N)]

def Yes(): print('Yes')

def No(): print('No')

def YES(): print('YES')

def NO(): print('NO')

sys.setrecursionlimit(10 ** 9)

INF = 10 ** 18

MOD = 10 ** 9 + 7



N = INT()

XY = []

for i in range(N):

    x, y = MAP()

    XY.append((x, y))



dist = 0

for i in range(N):

    x, y = XY[i]

    for j in range(i+1):

        x2, y2 = XY[j]

        dist += hypot(x-x2, y-y2)

print((dist * 2 / N))

# -*- coding: utf-8 -*-
import sys
from itertools import permutations
from math import hypot


def input():
    return sys.stdin.readline().strip()


def list2d(a, b, c):
    return [[c] * b for i in range(a)]


def list3d(a, b, c, d):
    return [[[d] * c for j in range(b)] for i in range(a)]


def list4d(a, b, c, d, e):
    return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]


def ceil(x, y=1):
    return int(-(-x // y))


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST(N=None):
    return list(MAP()) if N is None else [INT() for i in range(N)]


def Yes():
    print("Yes")


def No():
    print("No")


def YES():
    print("YES")


def NO():
    print("NO")


sys.setrecursionlimit(10**9)
INF = 10**18
MOD = 10**9 + 7
N = INT()
XY = []
for i in range(N):
    x, y = MAP()
    XY.append((x, y))
sm = cnt = 0
for perm in permutations(XY):
    dist = 0
    prev = (0, 0)
    for i, (x, y) in enumerate(perm):
        if i == 0:
            prev = (x, y)
        else:
            x2, y2 = prev
            dist += hypot(x - x2, y - y2)
    sm += dist
    cnt += 1
print((sm / cnt))

# -*- coding: utf-8 -*-
import sys
from math import hypot


def input():
    return sys.stdin.readline().strip()


def list2d(a, b, c):
    return [[c] * b for i in range(a)]


def list3d(a, b, c, d):
    return [[[d] * c for j in range(b)] for i in range(a)]


def list4d(a, b, c, d, e):
    return [[[[e] * d for j in range(c)] for j in range(b)] for i in range(a)]


def ceil(x, y=1):
    return int(-(-x // y))


def INT():
    return int(eval(input()))


def MAP():
    return list(map(int, input().split()))


def LIST(N=None):
    return list(MAP()) if N is None else [INT() for i in range(N)]


def Yes():
    print("Yes")


def No():
    print("No")


def YES():
    print("YES")


def NO():
    print("NO")


sys.setrecursionlimit(10**9)
INF = 10**18
MOD = 10**9 + 7
N = INT()
XY = []
for i in range(N):
    x, y = MAP()
    XY.append((x, y))
dist = 0
for i in range(N):
    x, y = XY[i]
    for j in range(i + 1):
        x2, y2 = XY[j]
        dist += hypot(x - x2, y - y2)
print((dist * 2 / N))

-from itertools import permutations -sm = cnt = 0 -for perm in permutations(XY): -    dist = 0 -    prev = (0, 0) -    for i, (x, y) in enumerate(perm): -        if i == 0: -            prev = (x, y) -        else: -            x2, y2 = prev -            dist += hypot(x - x2, y - y2) -    sm += dist -    cnt += 1 -print((sm / cnt)) +dist = 0 +for i in range(N): +    x, y = XY[i] +    for j in range(i + 1): +        x2, y2 = XY[j] +        dist += hypot(x - x2, y - y2) +print((dist * 2 / N))