import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()

# dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
# md = 10**9+7
md = 998244353

n = II()
to = [[] for _ in range(n)]
for _ in range(n-1):
    u, v = LI1()
    to[u].append(v)
    to[v].append(u)
cc = LI()

parent, depth, vis = [-1]*n, [0]*n, [0]*n

def dfs(root):
    uu = []
    vis[root] = 1
    stack = [root]
    while stack:
        iu = stack.pop()
        i, u = divmod(iu, n)
        if i == 0:
            uu.append(u)
        # care to's format
        while i < len(to[u]) and vis[to[u][i]]: i += 1
        if i < len(to[u]):
            stack.append((i+1)*n+u)
            # care to's format
            v = to[u][i]
            vis[v] = 1
            stack.append(v)
            parent[v] = u
            depth[v] = depth[u]+1

    return uu

uu = dfs(0)
cc = [c ^ 1 for c in cc]
dp = [0]*n

for u in uu[::-1]:
    if u==0:break
    p = parent[u]
    dp[p] += dp[u]
    if cc[u]:
        dp[p] += 1
        cc[p] ^= 1

if cc[0]:
    print(-1)
else:
    print(dp[0])