import sys
sys.setrecursionlimit(200050)

N = int(input())
AB = [list(map(int,input().split())) for _ in range(N-1)]
C = list(map(int,input().split()))
E = [[] for _ in range(N)]
for a,b in AB:
    a -= 1; b -= 1
    E[a].append(b)
    E[b].append(a)

ans = 0
def dfs(u,p):
    global ans
    cnt = 0
    for v in E[u]:
        if v != p:
            dfs(v,u)
            if C[v] == 0:
                cnt += 1
    if cnt % 2 == 1:
        C[u] ^= 1
    ans += cnt

dfs(0,-1)
if C[0] == 1:
    print(ans)
else:
    print(-1)