using System;
using System.IO;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using E = System.Linq.Enumerable;

partial class Solver
{
    struct Node
    {
        public int Index;
        public int DepthSum;
        public int DepthSumMin;
        public int Count;
    }

    Node Dfs(List<int>[] adj, string S, int pos, int pre)
    {
        var result = new Node { Index = pos, };
        if (S[pos] == '1')
        {
            result.Count++;
        }
        var children = new List<int>();
        foreach (var child in adj[pos])
        {
            if (child == pre) continue;
            var childNode = Dfs(adj, S, child, pos);
            result.Count += childNode.Count;
            result.DepthSum += childNode.DepthSum + childNode.Count;
            if (childNode.DepthSumMin + childNode.Count > 0)
            {
                children.Add(childNode.DepthSumMin + childNode.Count);
            }
        }

        if (children.Count >= 1)
        {
            var max = children.Max();
            var sum = children.Sum();
            var remain = sum - max;
            if (remain < max)
            {
                result.DepthSumMin = max - remain;
            }
            else
            {
                result.DepthSumMin = sum % 2;
            }
        }
        else
        {
            result.DepthSumMin = 0;
        }

        return result;
    }

    public void Run()
    {
        var N = ni();
        var S = ns();
        var adj = E.Range(0, N).Select(_ => new List<int>()).ToArray();
        for (int i = 0; i < N - 1; i++)
        {
            var a = ni() - 1;
            var b = ni() - 1;
            adj[a].Add(b);
            adj[b].Add(a);
        }
        int ans = int.MaxValue;

        for (int i = 0; i < N; i++)
        {
            var childNodes = new List<Node>();
            foreach (var child in adj[i])
            {
                var node = Dfs(adj, S, child, i);
                node.DepthSum += node.Count;
                node.DepthSumMin += node.Count;
                childNodes.Add(node);
            }
            var sum = childNodes.Sum(c => c.DepthSum);
            if (sum == 0)
            {
                ans = 0;
                continue;
            }
            if (sum % 2 == 1) continue;
            var maxChildDepth = childNodes.Max(c => c.DepthSum);
            var maxChildDepthNode = childNodes.First(c => c.DepthSum == maxChildDepth);
            var remain = sum - maxChildDepth;
            if (remain < sum)
            {
                if (maxChildDepthNode.DepthSumMin <= remain)
                {
                    ans = Math.Min(ans, sum / 2);
                }
            }
            else
            {
                ans = Math.Min(ans, sum / 2);
            }
        }
        if (ans == int.MaxValue) ans = -1;
        cout.WriteLine(ans);
    }
}

// PREWRITEN CODE BEGINS FROM HERE
partial class Solver : Scanner
{
    public static void Main(string[] args)
    {
#if LOCAL
        new Solver(Console.In, Console.Out).Run();
#else
        Console.SetOut(new StreamWriter(Console.OpenStandardOutput()) { AutoFlush = false });
        new Solver(Console.In, Console.Out).Run();
        Console.Out.Flush();
#endif
    }

#pragma warning disable IDE0052
    private readonly TextReader cin;
    private readonly TextWriter cout;
#pragma warning restore IDE0052

    public Solver(TextReader reader, TextWriter writer)
        : base(reader)
    {
        this.cin = reader;
        this.cout = writer;
    }
    public Solver(string input, TextWriter writer)
        : this(new StringReader(input), writer)
    {
    }

#pragma warning disable IDE1006
#pragma warning disable IDE0051
    private int ni() { return NextInt(); }
    private int[] ni(int n) { return NextIntArray(n); }
    private long nl() { return NextLong(); }
    private long[] nl(int n) { return NextLongArray(n); }
    private double nd() { return NextDouble(); }
    private double[] nd(int n) { return NextDoubleArray(n); }
    private string ns() { return Next(); }
    private string[] ns(int n) { return NextArray(n); }
#pragma warning restore IDE1006
#pragma warning restore IDE0051
}

public class Scanner
{
    private readonly TextReader Reader;
    private readonly Queue<string> TokenQueue = new Queue<string>();
    private readonly CultureInfo ci = CultureInfo.InvariantCulture;

    public Scanner()
        : this(Console.In)
    {
    }

    public Scanner(TextReader reader)
    {
        this.Reader = reader;
    }

    public int NextInt() { return int.Parse(Next(), ci); }
    public long NextLong() { return long.Parse(Next(), ci); }
    public double NextDouble() { return double.Parse(Next(), ci); }
    public string[] NextArray(int size)
    {
        var array = new string[size];
        for (int i = 0; i < size; i++) array[i] = Next();
        return array;
    }
    public int[] NextIntArray(int size)
    {
        var array = new int[size];
        for (int i = 0; i < size; i++) array[i] = NextInt();
        return array;
    }

    public long[] NextLongArray(int size)
    {
        var array = new long[size];
        for (int i = 0; i < size; i++) array[i] = NextLong();
        return array;
    }

    public double[] NextDoubleArray(int size)
    {
        var array = new double[size];
        for (int i = 0; i < size; i++) array[i] = NextDouble();
        return array;
    }

    public string Next()
    {
        if (TokenQueue.Count == 0)
        {
            if (!StockTokens()) throw new InvalidOperationException();
        }
        return TokenQueue.Dequeue();
    }

    public bool HasNext()
    {
        if (TokenQueue.Count > 0)
            return true;
        return StockTokens();
    }
    static readonly char[] _separator = new[] { ' ' };
    private bool StockTokens()
    {
        while (true)
        {
            var line = Reader.ReadLine();
            if (line == null) return false;
            var tokens = line.Split(_separator, StringSplitOptions.RemoveEmptyEntries);
            if (tokens.Length == 0) continue;
            foreach (var token in tokens)
                TokenQueue.Enqueue(token);
            return true;
        }
    }
}


Measured memory_bytes: 24952