#!/usr/bin/env python3

def main():

    from itertools import permutations

    from math import sqrt



    N = int(eval(input()))

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



    ans = 0

    cnt = 0

    for can_order in permutations(xy):

        cnt += 1

        for k in range(1, N):

            xi, yi = can_order[k - 1]

            xj, yj = can_order[k]

            ans += sqrt((xi - xj) ** 2 + (yi - yj) ** 2)

    # print(ans / cnt)

    print(f'{ans / cnt:.10f}')





if __name__ == '__main__':

    main()

#!/usr/bin/env python3

def main():

    from itertools import combinations

    from math import sqrt



    N = int(eval(input()))

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



    ans = 0

    for can_move in combinations(xy, 2):

        xi, yi = can_move[0]

        xj, yj = can_move[1]

        ans += sqrt((xi - xj) ** 2 + (yi - yj) ** 2)

    print(f'{ans * 2 / N:.10f}')





if __name__ == '__main__':

    main()

#!/usr/bin/env python3
def main():
    from itertools import permutations
    from math import sqrt

    N = int(eval(input()))
    xy = [list(map(int, input().split())) for _ in range(N)]
    ans = 0
    cnt = 0
    for can_order in permutations(xy):
        cnt += 1
        for k in range(1, N):
            xi, yi = can_order[k - 1]
            xj, yj = can_order[k]
            ans += sqrt((xi - xj) ** 2 + (yi - yj) ** 2)
    # print(ans / cnt)
    print(f"{ans / cnt:.10f}")


if __name__ == "__main__":
    main()

#!/usr/bin/env python3
def main():
    from itertools import combinations
    from math import sqrt

    N = int(eval(input()))
    xy = [list(map(int, input().split())) for _ in range(N)]
    ans = 0
    for can_move in combinations(xy, 2):
        xi, yi = can_move[0]
        xj, yj = can_move[1]
        ans += sqrt((xi - xj) ** 2 + (yi - yj) ** 2)
    print(f"{ans * 2 / N:.10f}")


if __name__ == "__main__":
    main()

-    from itertools import permutations +    from itertools import combinations -    cnt = 0 -    for can_order in permutations(xy): -        cnt += 1 -        for k in range(1, N): -            xi, yi = can_order[k - 1] -            xj, yj = can_order[k] -            ans += sqrt((xi - xj) ** 2 + (yi - yj) ** 2) -    # print(ans / cnt) -    print(f"{ans / cnt:.10f}") +    for can_move in combinations(xy, 2): +        xi, yi = can_move[0] +        xj, yj = can_move[1] +        ans += sqrt((xi - xj) ** 2 + (yi - yj) ** 2) +    print(f"{ans * 2 / N:.10f}")