n = int(input())
adj = [[] for _ in range(n)]
for i in range(n-1):
    a,b = map(int, input().split())
    adj[a-1].append(b-1)
    adj[b-1].append(a-1)
c = list(map(int, input().split()))
# for i in range(n): print(adj[i])

q = [(0, 0)]
pars = [-1] * n
route=[]
while q:
    v, par = q.pop()
    if pars[v] != -1: continue
    pars[v] = par
    route.append(v)
    for nv in adj[v]:
        if nv == par: continue
        q.append((nv, v))
# print(route)
# print(pars)

ans = 0

for v in reversed(route):
    if v == 0: continue
    par = pars[v]
    if not c[v] % 2:
        c[par] += 1
        ans += 1

# print(c)
if not c[0] % 2: print(-1)
else: print(ans)