import sys
sys.setrecursionlimit(10**6)
N = int(input())
G = {i:[] for i in range(1,N+1)}
for _ in range(N-1):
    a,b = map(int,input().split())
    G[a].append(b)
    G[b].append(a)
C = [-1]+list(map(int,input().split()))
cnt = 0
def dfs(x,p):
    global cnt
    for y in G[x]:
        if y==p:continue
        dfs(y,x)
    if x!=1 and C[x]==0:
        C[x] = 1
        C[p] = 1-C[p]
        cnt += 1
dfs(1,0)
if C[1]==0:
    print(-1)
else:
    print(cnt)