import sys

sys.setrecursionlimit(2147483647)

INF=float("inf")

MOD=10**9+7

input=lambda :sys.stdin.readline().rstrip()

def resolve():

    n=int(eval(input()))

    C=[0]*n

    for i in range(n):

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

        C[i]=complex(x,y)



    from itertools import product

    S=sum(abs(C[i]-C[j]) for i,j in product(list(range(n)),repeat=2))

    print((S/n))

resolve()
import sys

sys.setrecursionlimit(2147483647)

INF=float("inf")

MOD=10**9+7

input=lambda:sys.stdin.readline().rstrip()

def resolve():

    n=int(eval(input()))

    C=[None]*n

    for i in range(n):

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

        C[i]=x+y*1j



    s=sum(abs(C[i]-C[j]) for i in range(n) for j in range(n))

    print((s/n))

resolve()
import sys

sys.setrecursionlimit(2147483647)
INF = float("inf")
MOD = 10**9 + 7
input = lambda: sys.stdin.readline().rstrip()


def resolve():
    n = int(eval(input()))
    C = [0] * n
    for i in range(n):
        x, y = list(map(int, input().split()))
        C[i] = complex(x, y)
    from itertools import product

    S = sum(abs(C[i] - C[j]) for i, j in product(list(range(n)), repeat=2))
    print((S / n))


resolve()

import sys

sys.setrecursionlimit(2147483647)
INF = float("inf")
MOD = 10**9 + 7
input = lambda: sys.stdin.readline().rstrip()


def resolve():
    n = int(eval(input()))
    C = [None] * n
    for i in range(n):
        x, y = list(map(int, input().split()))
        C[i] = x + y * 1j
    s = sum(abs(C[i] - C[j]) for i in range(n) for j in range(n))
    print((s / n))


resolve()

-    C = [0] * n +    C = [None] * n -        C[i] = complex(x, y) -    from itertools import product - -    S = sum(abs(C[i] - C[j]) for i, j in product(list(range(n)), repeat=2)) -    print((S / n)) +        C[i] = x + y * 1j +    s = sum(abs(C[i] - C[j]) for i in range(n) for j in range(n)) +    print((s / n))