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

C = [0]+list(map(int, input().split()))
parent = [-1]*(N+1)
task = [1]
for v in task:
    for u in graph[v]:
        if parent[u] == -1:
            task.append(u)
            parent[u] = v

ans = 0
for v in task[1:][::-1]:
    if C[v]:
        continue
    C[parent[v]] ^= 1
    C[v] = 1
    ans += 1

print(ans if C[1] else -1)