import sys

import itertools

import math

n = int(sys.stdin.readline())



pos = [list(map(int, sys.stdin.readline().split())) for _ in range(n)]

a = list(itertools.permutations(pos, n))

#print(pos)

ans = 0

for ele in a:

    #print(ele)

    for i in range(n-1):

        ans += math.sqrt((ele[i][0] - ele[i+1][0])**2 + (ele[i][1] - ele[i+1][1])**2)

print((ans/len(a)))
#ABC145C

def main():

    import sys, math

    import itertools as ite

    N = int(sys.stdin.readline())

    L = [list(map(int, sys.stdin.readline().split())) for _ in range(N)]

    #print(L)

    

    A = list(ite.permutations([i for i in range(N)]))

    #print(A)

    ans = 0

    for a in A:

        for i in range(N-1):

            ans += math.sqrt((L[a[i]][0]- L[a[i+1]][0])**2 + (L[a[i]][1]- L[a[i+1]][1])**2)

    print((ans/len(A)))

            











if __name__=='__main__':

    main()
import sys
import itertools
import math

n = int(sys.stdin.readline())
pos = [list(map(int, sys.stdin.readline().split())) for _ in range(n)]
a = list(itertools.permutations(pos, n))
# print(pos)
ans = 0
for ele in a:
    # print(ele)
    for i in range(n - 1):
        ans += math.sqrt(
            (ele[i][0] - ele[i + 1][0]) ** 2 + (ele[i][1] - ele[i + 1][1]) ** 2
        )
print((ans / len(a)))

# ABC145C
def main():
    import sys, math
    import itertools as ite

    N = int(sys.stdin.readline())
    L = [list(map(int, sys.stdin.readline().split())) for _ in range(N)]
    # print(L)
    A = list(ite.permutations([i for i in range(N)]))
    # print(A)
    ans = 0
    for a in A:
        for i in range(N - 1):
            ans += math.sqrt(
                (L[a[i]][0] - L[a[i + 1]][0]) ** 2 + (L[a[i]][1] - L[a[i + 1]][1]) ** 2
            )
    print((ans / len(A)))


if __name__ == "__main__":
    main()

-import sys -import itertools -import math +# ABC145C +def main(): +    import sys, math +    import itertools as ite -n = int(sys.stdin.readline()) -pos = [list(map(int, sys.stdin.readline().split())) for _ in range(n)] -a = list(itertools.permutations(pos, n)) -# print(pos) -ans = 0 -for ele in a: -    # print(ele) -    for i in range(n - 1): -        ans += math.sqrt( -            (ele[i][0] - ele[i + 1][0]) ** 2 + (ele[i][1] - ele[i + 1][1]) ** 2 -        ) -print((ans / len(a))) +    N = int(sys.stdin.readline()) +    L = [list(map(int, sys.stdin.readline().split())) for _ in range(N)] +    # print(L) +    A = list(ite.permutations([i for i in range(N)])) +    # print(A) +    ans = 0 +    for a in A: +        for i in range(N - 1): +            ans += math.sqrt( +                (L[a[i]][0] - L[a[i + 1]][0]) ** 2 + (L[a[i]][1] - L[a[i + 1]][1]) ** 2 +            ) +    print((ans / len(A))) + + +if __name__ == "__main__": +    main()