# -*- coding: utf-8 -*-





def calc_dist(x1, x2, y1, y2):

    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5





def main():

    from itertools import permutations



    n = int(eval(input()))

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

    total = 0



    m = len(list(permutations(list(range(n)))))



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

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

            a = p[i]

            b = p[i + 1]

            total += calc_dist(xy[a][0], xy[b][0], xy[a][1], xy[b][1])



    print((total / m))





if __name__ == '__main__':

    main()

# -*- coding: utf-8 -*-





def main():

    from itertools import permutations

    from math import sqrt



    n = int(eval(input()))

    x = [0 for _ in range(n)]

    y = [0 for _ in range(n)]



    for i in range(n):

        xi, yi = list(map(int, input().split()))

        x[i] = xi

        y[i] = yi



    path = list(permutations(list(range(n))))

    path_count = len(path)

    length = 0



    for p in path:

        for i, j in zip(p, p[1:]):

            length += sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2)



    print((length / path_count))





if __name__ == '__main__':

    main()

# -*- coding: utf-8 -*-
def calc_dist(x1, x2, y1, y2):
    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5


def main():
    from itertools import permutations

    n = int(eval(input()))
    xy = [list(map(int, input().split())) for _ in range(n)]
    total = 0
    m = len(list(permutations(list(range(n)))))
    for p in permutations(list(range(n))):
        for i in range(len(p) - 1):
            a = p[i]
            b = p[i + 1]
            total += calc_dist(xy[a][0], xy[b][0], xy[a][1], xy[b][1])
    print((total / m))


if __name__ == "__main__":
    main()

# -*- coding: utf-8 -*-
def main():
    from itertools import permutations
    from math import sqrt

    n = int(eval(input()))
    x = [0 for _ in range(n)]
    y = [0 for _ in range(n)]
    for i in range(n):
        xi, yi = list(map(int, input().split()))
        x[i] = xi
        y[i] = yi
    path = list(permutations(list(range(n))))
    path_count = len(path)
    length = 0
    for p in path:
        for i, j in zip(p, p[1:]):
            length += sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2)
    print((length / path_count))


if __name__ == "__main__":
    main()

-def calc_dist(x1, x2, y1, y2): -    return ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5 - - +    from math import sqrt -    xy = [list(map(int, input().split())) for _ in range(n)] -    total = 0 -    m = len(list(permutations(list(range(n))))) -    for p in permutations(list(range(n))): -        for i in range(len(p) - 1): -            a = p[i] -            b = p[i + 1] -            total += calc_dist(xy[a][0], xy[b][0], xy[a][1], xy[b][1]) -    print((total / m)) +    x = [0 for _ in range(n)] +    y = [0 for _ in range(n)] +    for i in range(n): +        xi, yi = list(map(int, input().split())) +        x[i] = xi +        y[i] = yi +    path = list(permutations(list(range(n)))) +    path_count = len(path) +    length = 0 +    for p in path: +        for i, j in zip(p, p[1:]): +            length += sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) +    print((length / path_count))