N = int(input())
jisu = [0] * N
G = [set() for _ in range(N)]
for i in range(N - 1):
    u, v = map(int, input().split())
    u -= 1
    v -= 1
    G[u].add(v)
    G[v].add(u)
    jisu[u] += 1
    jisu[v] += 1
C = list(map(int, input().split()))

jisu1 = []
for i in range(N):
    if jisu[i] == 1:
        jisu1.append(i)

ans = 0
while len(jisu1) > 1:
    pos = jisu1.pop()

    jisu[pos] -= 1
    nex = G[pos].pop()
    G[nex].remove(pos)
    jisu[nex] -= 1
    if jisu[nex] == 1:
        jisu1.append(nex)

    # 色変える判断
    if C[pos] == 0:
        ans += 1
        C[nex] ^= 1

pos = jisu1.pop() # 最後の頂点
if C[pos] == 1:
    print(ans)
else:
    print(-1)