from itertools import permutations

from math import sqrt



n=int(eval(input()))

permlist = list(permutations(list(range(n))))

#print('permlist: {}'.format(permlist))



x = []

y = []

for i in range(n):

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

    x.append(xi)

    y.append(yi)



lengths = 0.0

for p in permlist:

    length = 0.0

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

        i = p[_]

        j = p[_+1]

        #print('i: {} j:{}'.format(i,j))

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

    #print('length: {}'.format(length))

    lengths += length



print((lengths/len(permlist)))

from itertools import permutations

from math import sqrt



def readinput():

    n=int(eval(input()))

    xyList=[]

    for _ in range(n):

        x,y=list(map(int,input().split()))

        xyList.append((x,y))

    return n,xyList



def main(n,xyList):

    sum=0.0

    factn=0

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

        #print(tup)

        factn+=1

        x0=xyList[tup[0]][0]

        y0=xyList[tup[0]][1]

        for i in tup:

            x=xyList[i][0]

            y=xyList[i][1]

            sum+=sqrt((x-x0)**2+(y-y0)**2)

            x0=x

            y0=y

    ans=sum/factn

    return ans



if __name__=='__main__':

    n,xyList=readinput()

    #print(xyList)

    ans=main(n,xyList)

    print(ans)



from itertools import permutations
from math import sqrt

n = int(eval(input()))
permlist = list(permutations(list(range(n))))
# print('permlist: {}'.format(permlist))
x = []
y = []
for i in range(n):
    xi, yi = list(map(int, input().split()))
    x.append(xi)
    y.append(yi)
lengths = 0.0
for p in permlist:
    length = 0.0
    for _ in range(len(p) - 1):
        i = p[_]
        j = p[_ + 1]
        # print('i: {} j:{}'.format(i,j))
        length += sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2)
    # print('length: {}'.format(length))
    lengths += length
print((lengths / len(permlist)))

from itertools import permutations
from math import sqrt


def readinput():
    n = int(eval(input()))
    xyList = []
    for _ in range(n):
        x, y = list(map(int, input().split()))
        xyList.append((x, y))
    return n, xyList


def main(n, xyList):
    sum = 0.0
    factn = 0
    for tup in permutations(list(range(n))):
        # print(tup)
        factn += 1
        x0 = xyList[tup[0]][0]
        y0 = xyList[tup[0]][1]
        for i in tup:
            x = xyList[i][0]
            y = xyList[i][1]
            sum += sqrt((x - x0) ** 2 + (y - y0) ** 2)
            x0 = x
            y0 = y
    ans = sum / factn
    return ans


if __name__ == "__main__":
    n, xyList = readinput()
    # print(xyList)
    ans = main(n, xyList)
    print(ans)

-n = int(eval(input())) -permlist = list(permutations(list(range(n)))) -# print('permlist: {}'.format(permlist)) -x = [] -y = [] -for i in range(n): -    xi, yi = list(map(int, input().split())) -    x.append(xi) -    y.append(yi) -lengths = 0.0 -for p in permlist: -    length = 0.0 -    for _ in range(len(p) - 1): -        i = p[_] -        j = p[_ + 1] -        # print('i: {} j:{}'.format(i,j)) -        length += sqrt((x[i] - x[j]) ** 2 + (y[i] - y[j]) ** 2) -    # print('length: {}'.format(length)) -    lengths += length -print((lengths / len(permlist))) + +def readinput(): +    n = int(eval(input())) +    xyList = [] +    for _ in range(n): +        x, y = list(map(int, input().split())) +        xyList.append((x, y)) +    return n, xyList + + +def main(n, xyList): +    sum = 0.0 +    factn = 0 +    for tup in permutations(list(range(n))): +        # print(tup) +        factn += 1 +        x0 = xyList[tup[0]][0] +        y0 = xyList[tup[0]][1] +        for i in tup: +            x = xyList[i][0] +            y = xyList[i][1] +            sum += sqrt((x - x0) ** 2 + (y - y0) ** 2) +            x0 = x +            y0 = y +    ans = sum / factn +    return ans + + +if __name__ == "__main__": +    n, xyList = readinput() +    # print(xyList) +    ans = main(n, xyList) +    print(ans)