N = int(input())
G = [[] for _ in range(N)]
for _ in range(N - 1):
    a,b = map(int,input().split())
    a -= 1
    b -= 1
    G[a].append(b)
    G[b].append(a)
c = list(map(int,input().split()))

stack = [(0,-1),(~0,-1)]
ans = 0
while stack:
    now,parent = stack.pop()
    if now < 0:
        now = ~now
        for v in G[now]:
            if v != parent:
                stack.append((v,now))
                stack.append((~v,now))
    else:
        if now == 0:
            if c[now] == 1:
                print(ans)
            else:
                print(-1)
            break
        else:
            if c[now] == 0:
                ans += 1
                c[now] = 1
                c[parent] ^= 1