I,R=input,range
def S(n):
	return int(n)-1
def J():
	return map(S,I().split())
N=int(I())
E=[set()for i in R(N)]
for i in R(N-1):
	a,b=J()
	E[a].add(b),E[b].add(a)
C=list(J())
L=set()
for i in R(N):
	if len(E[i])<2:L.add(i)
while len(L):
	M=set()
	for i in L:
		if len(E[i]):
			j=E[i].pop()
			C[j]+=C[i]
			E[j].remove(i)
			M.add(j)
	L=M
print([-C[j],-1][C[j]%2])