import java.io.*; import java.util.*; import java.util.stream.*; public class Main { static ArrayList> graph = new ArrayList<>(); static boolean[] isOn; static int ans; public static void main(String[] args) throws Exception { Scanner sc = new Scanner(); int n = sc.nextInt(); for (int i = 0; i < n; i++) { graph.add(new ArrayList<>()); } for (int i = 0; i < n - 1; i++) { int a = sc.nextInt() - 1; int b = sc.nextInt() - 1; graph.get(a).add(b); graph.get(b).add(a); } isOn = new boolean[n]; for (int i = 0; i < n; i++) { isOn[i] = (sc.nextInt() == 1); } if (getLight(0, 0)) { System.out.println(-1); } else { System.out.println(ans); } } static boolean getLight(int idx, int p) { for (int x : graph.get(idx)) { if (x != p) { isOn[idx] ^= getLight(x, idx); } } if (!isOn[idx]) { ans++; } return !isOn[idx]; } } class Utilities { static String arrayToLineString(Object[] arr) { return Arrays.stream(arr).map(x -> x.toString()).collect(Collectors.joining("\n")); } static String arrayToLineString(int[] arr) { return String.join("\n", Arrays.stream(arr).mapToObj(String::valueOf).toArray(String[]::new)); } } class Scanner { BufferedReader br = new BufferedReader(new InputStreamReader(System.in)); StringTokenizer st = new StringTokenizer(""); StringBuilder sb = new StringBuilder(); public Scanner() throws Exception { } public int nextInt() throws Exception { return Integer.parseInt(next()); } public long nextLong() throws Exception { return Long.parseLong(next()); } public double nextDouble() throws Exception { return Double.parseDouble(next()); } public int[] nextIntArray() throws Exception { return Stream.of(br.readLine().split(" ")).mapToInt(Integer::parseInt).toArray(); } public String next() throws Exception { while (!st.hasMoreTokens()) { st = new StringTokenizer(br.readLine()); } return st.nextToken(); } }