import sys

from itertools import permutations

from math import sqrt



def solve():

    input = sys.stdin.readline

    N = int(eval(input()))

    coordinate = dict()

    for i in range(N):

        coordinate[i] = tuple(map(int, input().split()))



    total_length = 0

    perm_count = 0

    for p in permutations(list(range(N))):

        perm_count += 1

        current_id = coordinate[int(p[0])]

        for j in range(1, N):

            next_id = coordinate[int(p[j])]

            total_length += sqrt((current_id[0] - next_id[0]) ** 2 + (current_id[1] - next_id[1]) ** 2)

            current_id = next_id



    print((total_length / perm_count))



    return 0



if __name__ == "__main__":

    solve()
import sys

from itertools import permutations

from math import sqrt



def solve():

    input = sys.stdin.readline

    N = int(eval(input()))

    C = [[int(i) for i in input().split()] for _ in range(N)]

    totalDist = 0

    count =0 

    for order in permutations(list(range(N))):

        count += 1

        dist = 0

        for i in range((N-1)):

            X = C[order[i]]

            Y = C[order[i+1]]

            dist += sqrt((X[0] - Y[0]) ** 2 + (X[1] - Y[1]) ** 2)

        totalDist += dist

    print((totalDist / count))



    return 0



if __name__ == "__main__":

    solve()
import sys
from itertools import permutations
from math import sqrt


def solve():
    input = sys.stdin.readline
    N = int(eval(input()))
    coordinate = dict()
    for i in range(N):
        coordinate[i] = tuple(map(int, input().split()))
    total_length = 0
    perm_count = 0
    for p in permutations(list(range(N))):
        perm_count += 1
        current_id = coordinate[int(p[0])]
        for j in range(1, N):
            next_id = coordinate[int(p[j])]
            total_length += sqrt(
                (current_id[0] - next_id[0]) ** 2 + (current_id[1] - next_id[1]) ** 2
            )
            current_id = next_id
    print((total_length / perm_count))
    return 0


if __name__ == "__main__":
    solve()

import sys
from itertools import permutations
from math import sqrt


def solve():
    input = sys.stdin.readline
    N = int(eval(input()))
    C = [[int(i) for i in input().split()] for _ in range(N)]
    totalDist = 0
    count = 0
    for order in permutations(list(range(N))):
        count += 1
        dist = 0
        for i in range((N - 1)):
            X = C[order[i]]
            Y = C[order[i + 1]]
            dist += sqrt((X[0] - Y[0]) ** 2 + (X[1] - Y[1]) ** 2)
        totalDist += dist
    print((totalDist / count))
    return 0


if __name__ == "__main__":
    solve()

-    coordinate = dict() -    for i in range(N): -        coordinate[i] = tuple(map(int, input().split())) -    total_length = 0 -    perm_count = 0 -    for p in permutations(list(range(N))): -        perm_count += 1 -        current_id = coordinate[int(p[0])] -        for j in range(1, N): -            next_id = coordinate[int(p[j])] -            total_length += sqrt( -                (current_id[0] - next_id[0]) ** 2 + (current_id[1] - next_id[1]) ** 2 -            ) -            current_id = next_id -    print((total_length / perm_count)) +    C = [[int(i) for i in input().split()] for _ in range(N)] +    totalDist = 0 +    count = 0 +    for order in permutations(list(range(N))): +        count += 1 +        dist = 0 +        for i in range((N - 1)): +            X = C[order[i]] +            Y = C[order[i + 1]] +            dist += sqrt((X[0] - Y[0]) ** 2 + (X[1] - Y[1]) ** 2) +        totalDist += dist +    print((totalDist / count))