using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.IO;
using System.Diagnostics;


struct Myon2
{
    int a;
}

class Myon
{
    public Myon() { }
    public static int Main()
    {
        new Myon().calc();
        return 0;
    }

    Scanner cin;


    List<int>[] es;
    int N;
    string S;
    int[] parent;


    void calc()
    {
        cin = new Scanner();
        N = cin.nextInt();
        S = cin.next();
        es = new List<int>[N];
        for (int i = 0; i < N; i++)
        {
            es[i] = new List<int>();
        }

        for (int i = 0; i < N - 1; i++)
        {
            int a = cin.nextInt() - 1;
            int b = cin.nextInt() - 1;
            es[a].Add(b);
            es[b].Add(a);
        }

        int MAX = int.MaxValue / 2;
        ans = MAX;


        for (int i = 0; i < N; i++)
        {

            childCoin = new int[N];
            childSumLen = new long[N];
            childMinLen = new long[N];
            childMaxID = new int[N];
            parent = new int[N];

            dfs(i, -1);
            if (childMinLen[i] == 0)
            {

                ans = Math.Min(ans, childSumLen[i] / 2);
            }
        }

        if (ans == MAX) ans = -1;
        Console.WriteLine(ans);
    }

    int[] childCoin;
    long[] childSumLen;
    long[] childMinLen;
    int[] childMaxID;


    long ans;



    void dfs(int now, int pre)
    {
        parent[now] = pre;
        foreach (var next in es[now])
        {
            if (next == parent[now]) continue;
            dfs(next, now);
        }

        int maxID = -1;
        long maxMinLen = 0;


        foreach (var next in es[now])
        {
            if (next == parent[now]) continue;

            childCoin[now] += childCoin[next];
            childSumLen[now] += childSumLen[next] + childCoin[next];
            if (maxID == -1 || maxMinLen < childMinLen[next] + childCoin[next])
            {
                maxID = next;
                maxMinLen = childMinLen[next] + childCoin[next];
            }
        }
        childMaxID[now] = maxID;

        childMinLen[now] = maxMinLen;

        foreach (var next in es[now])
        {
            if (next == parent[now]) continue;
            if (next == maxID) continue;
            childMinLen[now] -= childSumLen[next] + childCoin[next];
        }

        childMinLen[now] = Math.Max(childMinLen[now], childSumLen[now] % 2);

        if (S[now] == '1') childCoin[now]++;

        //Console.WriteLine($"{now} {childCoin[now]} {childMinLen[now]} {childSumLen[now]}");
    }

}




class Scanner
{
    string[] s;
    int i;

    char[] cs = new char[] { ' ' };

    public Scanner()
    {
        s = new string[0];
        i = 0;
    }

    public string next()
    {
        if (i < s.Length) return s[i++];
        string st = Console.ReadLine();
        while (st == "") st = Console.ReadLine();
        s = st.Split(cs, StringSplitOptions.RemoveEmptyEntries);
        if (s.Length == 0) return next();
        i = 0;
        return s[i++];
    }

    public int nextInt()
    {
        return int.Parse(next());
    }
    public int[] ArrayInt(int N, int add = 0)
    {
        int[] Array = new int[N];
        for (int i = 0; i < N; i++)
        {
            Array[i] = nextInt() + add;
        }
        return Array;
    }

    public long nextLong()
    {
        return long.Parse(next());
    }

    public long[] ArrayLong(int N, long add = 0)
    {
        long[] Array = new long[N];
        for (int i = 0; i < N; i++)
        {
            Array[i] = nextLong() + add;
        }
        return Array;
    }

    public double nextDouble()
    {
        return double.Parse(next());
    }


    public double[] ArrayDouble(int N, double add = 0)
    {
        double[] Array = new double[N];
        for (int i = 0; i < N; i++)
        {
            Array[i] = nextDouble() + add;
        }
        return Array;
    }
}


Measured memory_bytes: 37712