div1-medium

from math import factorial as fact

def binom(n, k):
    return fact(n) // fact(k) // fact(n - k)


def dfs(v, sz, a, b, c, d, g, w):
    sz[v] = 1
    a[v][1] = 1
    b[v][1] = w[v]
    d[v][1] = w[v] * w[v]
    c[v][1] = 0

    for to in g[v]:
        dfs(to, sz, a, b, c, d, g, w)
        for k in range(sz[v], 0, -1):
            for i in range(sz[to], 0, -1):
                a[v][k + i] += a[v][k] * a[to][i]

                b[v][k + i] += b[v][k] * a[to][i] + b[to][i] * a[v][k]
                d[v][k + i] += d[v][k] * a[to][i] + d[to][i] * a[v][k]

                c[v][k + i] += c[v][k] * a[to][i] + c[to][i] * a[v][k]
                c[v][k + i] += b[v][k] * b[to][i]

        sz[v] += sz[to]


def calc(n):
    c1 = [0] * (n + 1)
    c2 = [0] * (n + 1)
    for m in range(1, n + 1):
        c1[m] = m ** (-1) - m ** (-2)

    for m in range(1, n + 1):
        c2[m] = -2  * (m ** (-2))

    return c1, c2


class AverageVarianceSubtree:
    def average(self, p, w):
        n = len(w)
        g = [[] for _ in range(n)]
        for v in range(1, n):
            g[p[v - 1]].append(v)

        sz = [0] * n
        a = [[0] * (n + 1) for _ in range(n)]
        b = [[0] * (n + 1) for _ in range(n)]
        c = [[0] * (n + 1) for _ in range(n)]
        d = [[0] * (n + 1) for _ in range(n)]

        root = 0
        dfs(root, sz, a, b, c, d, g, w)
        c1, c2 = calc(n)

        ans = 0
        tot = 0
        for v in range(n):
            for m in range(1, sz[v] + 1):
                tot += a[v][m]
                ans += c1[m] * d[v][m] + c2[m] * c[v][m]
                print(v, m)
                print(d[v][m], c[v][m])
                print()


        print('tot = ', tot)

        return ans / tot



if __name__ == '__main__':
    solver = AverageVarianceSubtree()
    p = list(map(int, input().split()))
    w = list(map(int, input().split()))
    print(solver.average(p, w))