def non_rec_dfs(s):
    stack = []
    stack.append(~s)
    stack.append(s)
    par = [-1] * N
    ans = 0
    while stack:
        u = stack.pop()
        if u >= 0:
            stack.append(~u)
            for v in G[u]:
                if v == par[u]:
                    continue
                par[v] = u
                stack.append(v)
        else:
            u = ~u
            if C[u] == 0:
                C[u] = 1
                C[par[u]] = 1 - C[par[u]]
                ans += 1
    return ans

N = int(input())
G = [[] for i in range(N)]
for i in range(N - 1):
    A, B = map(int, input().split())
    A, B = A - 1, B - 1
    G[A].append(B)
    G[B].append(A)
    
C = list(map(int, input().split()))
ans = non_rec_dfs(0)
print(ans) if sum(C) == N else print(-1)