def main():

    N = int(eval(input()))

    ans = 0

    from itertools import permutations

    from math import hypot

    XY = [[int(i) for i in input().split()] for j in range(N)]

    for p in permutations(list(range(N))):

        cur = 0

        for i in range(N-1):

            cur += hypot(XY[p[i]][0] - XY[p[i+1]][0],

                         XY[p[i]][1] - XY[p[i+1]][1])

        ans += cur

    fact = 1

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

        fact *= i

    print((ans / fact))





if __name__ == '__main__':

    main()

def main():

    import sys

    input = sys.stdin.buffer.readline

    from itertools import permutations

    from math import hypot

    N = int(eval(input()))

    XY = [[int(i) for i in input().split()] for j in range(N)]

    ans = 0

    for p in permutations(list(range(N))):

        for i in range(N-1):

            ans += hypot(XY[p[i+1]][0] - XY[p[i]][0],

                         XY[p[i+1]][1] - XY[p[i]][1])

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

        ans /= i

    print(ans)





if __name__ == '__main__':

    main()

def main():
    N = int(eval(input()))
    ans = 0
    from itertools import permutations
    from math import hypot

    XY = [[int(i) for i in input().split()] for j in range(N)]
    for p in permutations(list(range(N))):
        cur = 0
        for i in range(N - 1):
            cur += hypot(XY[p[i]][0] - XY[p[i + 1]][0], XY[p[i]][1] - XY[p[i + 1]][1])
        ans += cur
    fact = 1
    for i in range(1, N + 1):
        fact *= i
    print((ans / fact))


if __name__ == "__main__":
    main()

def main():
    import sys

    input = sys.stdin.buffer.readline
    from itertools import permutations
    from math import hypot

    N = int(eval(input()))
    XY = [[int(i) for i in input().split()] for j in range(N)]
    ans = 0
    for p in permutations(list(range(N))):
        for i in range(N - 1):
            ans += hypot(XY[p[i + 1]][0] - XY[p[i]][0], XY[p[i + 1]][1] - XY[p[i]][1])
    for i in range(1, N + 1):
        ans /= i
    print(ans)


if __name__ == "__main__":
    main()

-    N = int(eval(input())) -    ans = 0 +    import sys + +    input = sys.stdin.buffer.readline +    N = int(eval(input())) +    ans = 0 -        cur = 0 -            cur += hypot(XY[p[i]][0] - XY[p[i + 1]][0], XY[p[i]][1] - XY[p[i + 1]][1]) -        ans += cur -    fact = 1 +            ans += hypot(XY[p[i + 1]][0] - XY[p[i]][0], XY[p[i + 1]][1] - XY[p[i]][1]) -        fact *= i -    print((ans / fact)) +        ans /= i +    print(ans)