import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
n = ni()
g = [[] for i in range(n)]
deg = [0] * n
for _ in range(n-1):
    a,b = na()
    a-=1
    b-=1
    g[a].append(b)
    g[b].append(a)
    deg[a] += 1
    deg[b] += 1
c = [i^1 for i in na()]
p = [i for i in range(n) if deg[i] == 1]
ans = 0
while p:
    x = p.pop()
    print(p, x)
    deg[x] -= 1
    for y in g[x]:
        if deg[y] >= 1:
            deg[y] -= 1
        if deg[y] == 1:
            p.append(y)
        if deg[y] >= 1 and c[x] == 1:
            c[x] ^= 1
            c[y] ^= 1
            ans += 1
if sum(c) == 2:
    print(ans+1)
elif sum(c) == 1:
    print(-1)
else:
    print(ans)