using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    static int[] NMi => ReadLine().Split().Select(c => int.Parse(c) - 1).ToArray();
    static int[][] NMap(int n) => Enumerable.Repeat(0, n).Select(_ => NMi).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = int.Parse(ReadLine());
        var map = NMap(n - 1);
        var c = NList;
        var tree = new List<int>[n];
        for (var i = 0; i < tree.Length; ++i) tree[i] = new List<int>();
        foreach (var edge in map)
        {
            tree[edge[0]].Add(edge[1]);
            tree[edge[1]].Add(edge[0]);
        }
        var colors = c.Select(ci => ci == 0).ToArray();
        var ans = 0;
        DFS(-1, 0, tree, colors, ref ans);
        WriteLine(colors[0] ? -1 : ans);
    }
    static void DFS(int prev, int cur, List<int>[] tree, bool[] colors, ref int ans)
    {
        foreach (var next in tree[cur])
        {
            if (prev == next) continue;
            DFS(cur, next, tree, colors, ref ans);
            if (colors[next])
            {
                ++ans;
                colors[next] = false;
                colors[cur] = !colors[cur];
            }
        }
    }
}