def main():

    global visited, adj, sets

    print = out.append

    ''' Cook your dish here! '''

    q = get_int()

    points = []

    res = 0

    for _ in range(q):

        pt = get_list()

        for i in points:

            res += ((pt[0]-i[0])**2+(pt[1]-i[1])**2)**(1/2)

        points.append(pt)

    print(2*res/q)





''' Coded with love at India by Satyam Kumar '''

import sys, threading, math

from collections import defaultdict

input = iter(sys.stdin.buffer.read().decode().splitlines()).__next__

out = []

get_int = lambda: int(input())

get_list = lambda: list(map(int, input().split()))

main()

# [main() for _ in range(int(input()))]

print(*out, sep='\n')

def main():

    global visited, adj, sets

    print = out.append

    ''' Cook your dish here! '''

    q = get_int()

    points = []

    res = 0

    for _ in range(q):

        pt = get_list()

        for i in points:

            res += ((pt[0]-i[0])**2+(pt[1]-i[1])**2)**(1/2)

        points.append(pt)

    print(2*res/q)





''' Coded with love at India by Satyam Kumar '''

import sys

#from collections import defaultdict

input = iter(sys.stdin.buffer.read().decode().splitlines()).__next__

out = []

get_int = lambda: int(input())

get_list = lambda: list(map(int, input().split()))

main()

# [main() for _ in range(int(input()))]

print(*out, sep='\n')

def main():
    global visited, adj, sets
    print = out.append
    """ Cook your dish here! """
    q = get_int()
    points = []
    res = 0
    for _ in range(q):
        pt = get_list()
        for i in points:
            res += ((pt[0] - i[0]) ** 2 + (pt[1] - i[1]) ** 2) ** (1 / 2)
        points.append(pt)
    print(2 * res / q)


""" Coded with love at India by Satyam Kumar """
import sys, threading, math
from collections import defaultdict

input = iter(sys.stdin.buffer.read().decode().splitlines()).__next__
out = []
get_int = lambda: int(input())
get_list = lambda: list(map(int, input().split()))
main()
# [main() for _ in range(int(input()))]
print(*out, sep="\n")

def main():
    global visited, adj, sets
    print = out.append
    """ Cook your dish here! """
    q = get_int()
    points = []
    res = 0
    for _ in range(q):
        pt = get_list()
        for i in points:
            res += ((pt[0] - i[0]) ** 2 + (pt[1] - i[1]) ** 2) ** (1 / 2)
        points.append(pt)
    print(2 * res / q)


""" Coded with love at India by Satyam Kumar """
import sys

# from collections import defaultdict
input = iter(sys.stdin.buffer.read().decode().splitlines()).__next__
out = []
get_int = lambda: int(input())
get_list = lambda: list(map(int, input().split()))
main()
# [main() for _ in range(int(input()))]
print(*out, sep="\n")

-import sys, threading, math -from collections import defaultdict +import sys +# from collections import defaultdict