import itertools



n = int(eval(input()))

town = [tuple(map(int,input().split())) for _ in range(n)]

c_town = list(itertools.combinations(town,2))

lp = len(list(itertools.permutations(town)))

sum_road = 0



for i in c_town:

  sum_road += ((i[0][0]-i[1][0])**2 + (i[0][1]-i[1][1])**2)**0.5

  

print((2*sum_road/n))
import math

from itertools import combinations



N = int(eval(input()))

xy = [tuple(map(int,input().split())) for _ in range(N)]



roots = list(combinations(list(range(0,N)),2))



sum_r = 0

for t, nt in roots:

  sum_r += math.sqrt((xy[t][0] - xy[nt][0])**2 + (xy[t][1] - xy[nt][1])**2)

  

res = 2*sum_r/N

print(res)
import itertools

n = int(eval(input()))
town = [tuple(map(int, input().split())) for _ in range(n)]
c_town = list(itertools.combinations(town, 2))
lp = len(list(itertools.permutations(town)))
sum_road = 0
for i in c_town:
    sum_road += ((i[0][0] - i[1][0]) ** 2 + (i[0][1] - i[1][1]) ** 2) ** 0.5
print((2 * sum_road / n))

import math
from itertools import combinations

N = int(eval(input()))
xy = [tuple(map(int, input().split())) for _ in range(N)]
roots = list(combinations(list(range(0, N)), 2))
sum_r = 0
for t, nt in roots:
    sum_r += math.sqrt((xy[t][0] - xy[nt][0]) ** 2 + (xy[t][1] - xy[nt][1]) ** 2)
res = 2 * sum_r / N
print(res)

-import itertools +import math +from itertools import combinations -n = int(eval(input())) -town = [tuple(map(int, input().split())) for _ in range(n)] -c_town = list(itertools.combinations(town, 2)) -lp = len(list(itertools.permutations(town))) -sum_road = 0 -for i in c_town: -    sum_road += ((i[0][0] - i[1][0]) ** 2 + (i[0][1] - i[1][1]) ** 2) ** 0.5 -print((2 * sum_road / n)) +N = int(eval(input())) +xy = [tuple(map(int, input().split())) for _ in range(N)] +roots = list(combinations(list(range(0, N)), 2)) +sum_r = 0 +for t, nt in roots: +    sum_r += math.sqrt((xy[t][0] - xy[nt][0]) ** 2 + (xy[t][1] - xy[nt][1]) ** 2) +res = 2 * sum_r / N +print(res)