using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Collections.Generic;
using System.Diagnostics;
using Enu = System.Linq.Enumerable;
using System.Numerics;

public class Program
{
    long ans = 0, TotalA, TotalB;
    int[] Val;
    long[] SumA, SumB;
    List<int>[] G;

    public void Solve()
    {
        int N = Reader.Int();
        Val = Reader.String().Select(c => c == 'c' ? 0 : 1).ToArray();
        G = Enu.Range(0, N).Select(i => new List<int>()).ToArray();
        for (int i = 0; i < N - 1; i++)
        {
            int a = Reader.Int() - 1, b = Reader.Int() - 1;
            G[a].Add(b);
            G[b].Add(a);
        }
        SumA = new long[N];
        SumB = new long[N];
        TotalA = Val.Count(c => c == 0);
        TotalB = N - TotalA;
        Rec(0, 0);
        Console.WriteLine(ans);
    }

    private void Rec(int at, int prev)
    {
        foreach (int next in G[at])
            if (next != prev)
            {
                Rec(next, at);
                SumA[at] += SumA[next];
                SumB[at] += SumB[next];
                if (Val[at] == 1)
                    ans += SumA[next] * (TotalB - 1 - SumB[next]);
            }
        if (Val[at] == 0) SumA[at]++;
        else { ans += (TotalA - SumA[at]) * SumB[at]; SumB[at]++; }
    }


}


class Entry { static void Main() { new Program().Solve(); } }
class Reader
{
    static TextReader reader = Console.In;
    static readonly char[] separator = { ' ' };
    static readonly StringSplitOptions op = StringSplitOptions.RemoveEmptyEntries;
    static string[] A = new string[0];
    static int i;
    static void Init() { A = new string[0]; }
    public static void Set(TextReader r) { reader = r; Init(); }
    public static void Set(string file) { reader = new StreamReader(file); Init(); }
    public static bool HasNext() { return CheckNext(); }
    public static string String() { return Next(); }
    public static int Int() { return int.Parse(Next()); }
    public static long Long() { return long.Parse(Next()); }
    public static double Double() { return double.Parse(Next()); }
    public static int[] IntLine() { return Array.ConvertAll(Split(Line()), int.Parse); }
    public static int[] IntArray(int N) { return Range(N, Int); }
    public static int[][] IntTable(int H) { return Range(H, IntLine); }
    public static string[] StringArray(int N) { return Range(N, Next); }
    public static string[][] StringTable(int N) { return Range(N, () => Split(Line())); }
    public static string Line() { return reader.ReadLine().Trim(); }
    static string[] Split(string s) { return s.Split(separator, op); }
    static T[] Range<T>(int N, Func<T> f) { var r = new T[N]; for (int i = 0; i < N; r[i++] = f()) ; return r; }
    static string Next() { CheckNext(); return A[i++]; }
    static bool CheckNext()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return CheckNext();
        A = Split(line);
        i = 0;
        return true;
    }
}