import math



N = int(eval(input()))

x = [0]*N

y = [0]*N

for i in range(N):

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

    

ans=0

for i in range(N):

    for j in range(i):

        ans+=math.sqrt((x[i]-x[j])**2+(y[i]-y[j])**2)

print((ans*2/N))
import sys

input = sys.stdin.readline



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

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

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



def main():

    mod=10**9+7

    N=I()

    X=[0]*N

    Y=[0]*N

    d=[[0]*N for _ in range(N)]

    for i in range(N):

        X[i],Y[i]=MI()

        

    for i in range(N):

        for j in range(N):

            dx=X[i]-X[j]

            dy=Y[i]-Y[j]

            d[i][j]=(dx**2 + dy**2)**0.5

            

    import itertools

    

    ans=0

    for ite in itertools.permutations(list(range(N)), N):

        now=ite[0]

        for i in range(1,N):

            nxt=ite[i]

            ans+=d[nxt][now]

            now=nxt

    M=1

    for i in range(1,N+1):

        M*=i

    ans=ans/M

    print(ans)

            



main()

import math

N = int(eval(input()))
x = [0] * N
y = [0] * N
for i in range(N):
    x[i], y[i] = list(map(int, input().split()))
ans = 0
for i in range(N):
    for j in range(i):
        ans += math.sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2)
print((ans * 2 / N))

import sys

input = sys.stdin.readline


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


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


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


def main():
    mod = 10**9 + 7
    N = I()
    X = [0] * N
    Y = [0] * N
    d = [[0] * N for _ in range(N)]
    for i in range(N):
        X[i], Y[i] = MI()
    for i in range(N):
        for j in range(N):
            dx = X[i] - X[j]
            dy = Y[i] - Y[j]
            d[i][j] = (dx**2 + dy**2) ** 0.5
    import itertools

    ans = 0
    for ite in itertools.permutations(list(range(N)), N):
        now = ite[0]
        for i in range(1, N):
            nxt = ite[i]
            ans += d[nxt][now]
            now = nxt
    M = 1
    for i in range(1, N + 1):
        M *= i
    ans = ans / M
    print(ans)


main()

-import math +import sys -N = int(eval(input())) -x = [0] * N -y = [0] * N -for i in range(N): -    x[i], y[i] = list(map(int, input().split())) -ans = 0 -for i in range(N): -    for j in range(i): -        ans += math.sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) -print((ans * 2 / N)) +input = sys.stdin.readline + + +def I(): +    return int(eval(input())) + + +def MI(): +    return list(map(int, input().split())) + + +def LI(): +    return list(map(int, input().split())) + + +def main(): +    mod = 10**9 + 7 +    N = I() +    X = [0] * N +    Y = [0] * N +    d = [[0] * N for _ in range(N)] +    for i in range(N): +        X[i], Y[i] = MI() +    for i in range(N): +        for j in range(N): +            dx = X[i] - X[j] +            dy = Y[i] - Y[j] +            d[i][j] = (dx**2 + dy**2) ** 0.5 +    import itertools + +    ans = 0 +    for ite in itertools.permutations(list(range(N)), N): +        now = ite[0] +        for i in range(1, N): +            nxt = ite[i] +            ans += d[nxt][now] +            now = nxt +    M = 1 +    for i in range(1, N + 1): +        M *= i +    ans = ans / M +    print(ans) + + +main()