import itertools



N = int(eval(input()))

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

total_distance = 0

total_count = 0



for route in itertools.permutations(dots):

    for i in range(len(route)-1):

        x1, y1 = route[i]

        x2, y2 = route[i+1]



        total_distance += ((x2 - x1)**2 + (y2 - y1)**2)**0.5

    

    total_count += 1



ans = total_distance / total_count

print(ans)





# import math



N = int(eval(input()))

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



total_distance = 0

count = 0

for start in range(N-1):

    for end in range(start+1, N):

        start_x, start_y = towns[start]

        end_x, end_y = towns[end]



        total_distance += ((end_x - start_x)**2 + (end_y - start_y)**2)**0.5

        count += 1



# factor = math.factorial(N)



# answer = total_distance * (factor * (N - 1) / count) / factor

answer = total_distance * ((N - 1) / count)

print(answer)
import itertools

N = int(eval(input()))
dots = [tuple(map(int, input().split())) for _ in range(N)]
total_distance = 0
total_count = 0
for route in itertools.permutations(dots):
    for i in range(len(route) - 1):
        x1, y1 = route[i]
        x2, y2 = route[i + 1]
        total_distance += ((x2 - x1) ** 2 + (y2 - y1) ** 2) ** 0.5
    total_count += 1
ans = total_distance / total_count
print(ans)

# import math
N = int(eval(input()))
towns = [tuple(map(int, input().split())) for _ in range(N)]
total_distance = 0
count = 0
for start in range(N - 1):
    for end in range(start + 1, N):
        start_x, start_y = towns[start]
        end_x, end_y = towns[end]
        total_distance += ((end_x - start_x) ** 2 + (end_y - start_y) ** 2) ** 0.5
        count += 1
# factor = math.factorial(N)
# answer = total_distance * (factor * (N - 1) / count) / factor
answer = total_distance * ((N - 1) / count)
print(answer)

-import itertools - +# import math -dots = [tuple(map(int, input().split())) for _ in range(N)] +towns = [tuple(map(int, input().split())) for _ in range(N)] -total_count = 0 -for route in itertools.permutations(dots): -    for i in range(len(route) - 1): -        x1, y1 = route[i] -        x2, y2 = route[i + 1] -        total_distance += ((x2 - x1) ** 2 + (y2 - y1) ** 2) ** 0.5 -    total_count += 1 -ans = total_distance / total_count -print(ans) +count = 0 +for start in range(N - 1): +    for end in range(start + 1, N): +        start_x, start_y = towns[start] +        end_x, end_y = towns[end] +        total_distance += ((end_x - start_x) ** 2 + (end_y - start_y) ** 2) ** 0.5 +        count += 1 +# factor = math.factorial(N) +# answer = total_distance * (factor * (N - 1) / count) / factor +answer = total_distance * ((N - 1) / count) +print(answer)