from collections import deque
n = int(input())
adj = [[] for _ in range(n)]
for _ 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()))
if c.count(0)%2: print(-1); exit()
dq = deque([0]); topo = []; par = [-1]*n; chi = [[] for _ in range(n)]
while dq:
    p = dq.popleft(); topo.append(p)
    for v in adj[p]:
        if v!=par[p]: dq.append(v); par[v] = p; chi[p].append(v)
ans = 0
for v in topo[::-1]:
    if not c[v]: c[v] ^= 1; c[par[v]] ^= 1; ans += 1
print(ans)