from itertools import permutations

from functools import reduce

from operator import mul





def solve(string):

    n, *xy = list(map(int, string.split()))

    *xy, = list(zip(*[iter(xy)] * 2))

    return str(

        sum(

            sum(((xy[p0][0] - xy[p1][0])**2 + (xy[p0][1] - xy[p1][1])**2)**0.5

                for p0, p1 in zip(list(i),

                                  list(i)[1:]))

            for i in permutations(list(range(n)), n)) / reduce(mul, list(range(1, n + 1))))





if __name__ == '__main__':

    n = int(eval(input()))

    print((solve('{}\n'.format(n)+'\n'.join([eval(input()) for _ in range(n)]))))

from itertools import product





def solve(string):

    n, *xy = list(map(int, string.split()))

    *xy, = list(zip(*[iter(xy)] * 2))

    return str(

        sum(((xy[i][0] - xy[j][0])**2 + (xy[i][1] - xy[j][1])**2)**0.5

            for i, j in product(list(range(n)), repeat=2)) / n)





if __name__ == '__main__':

    n = int(eval(input()))

    print((solve('{}\n'.format(n) + '\n'.join([eval(input()) for _ in range(n)]))))

from itertools import permutations
from functools import reduce
from operator import mul


def solve(string):
    n, *xy = list(map(int, string.split()))
    (*xy,) = list(zip(*[iter(xy)] * 2))
    return str(
        sum(
            sum(
                ((xy[p0][0] - xy[p1][0]) ** 2 + (xy[p0][1] - xy[p1][1]) ** 2) ** 0.5
                for p0, p1 in zip(list(i), list(i)[1:])
            )
            for i in permutations(list(range(n)), n)
        )
        / reduce(mul, list(range(1, n + 1)))
    )


if __name__ == "__main__":
    n = int(eval(input()))
    print((solve("{}\n".format(n) + "\n".join([eval(input()) for _ in range(n)]))))

from itertools import product


def solve(string):
    n, *xy = list(map(int, string.split()))
    (*xy,) = list(zip(*[iter(xy)] * 2))
    return str(
        sum(
            ((xy[i][0] - xy[j][0]) ** 2 + (xy[i][1] - xy[j][1]) ** 2) ** 0.5
            for i, j in product(list(range(n)), repeat=2)
        )
        / n
    )


if __name__ == "__main__":
    n = int(eval(input()))
    print((solve("{}\n".format(n) + "\n".join([eval(input()) for _ in range(n)]))))

-from itertools import permutations -from functools import reduce -from operator import mul +from itertools import product -            sum( -                ((xy[p0][0] - xy[p1][0]) ** 2 + (xy[p0][1] - xy[p1][1]) ** 2) ** 0.5 -                for p0, p1 in zip(list(i), list(i)[1:]) -            ) -            for i in permutations(list(range(n)), n) +            ((xy[i][0] - xy[j][0]) ** 2 + (xy[i][1] - xy[j][1]) ** 2) ** 0.5 +            for i, j in product(list(range(n)), repeat=2) -        / reduce(mul, list(range(1, n + 1))) +        / n