import java.io.*;
import java.util.*;
import java.util.stream.*;

public class Main {
    static ArrayList<ArrayList<Integer>> 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();
    }
    
}