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;
    int[] Val;
    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);
        }
        var who = new List<int>();
        var vals = new List<int>();
        var last = new int[N];
        Rec(0, -1, G, Val, who, vals, last);
        var As = new BinaryIndexedTree(N * 2);
        var Bs = new BinaryIndexedTree(N * 2);
        long totalA = vals.Count(c => c == 0);
        long totalB = N - totalA;

        for (int i = 0; i < vals.Count; i++)
            if (vals[i] == 0) As.Add(i, 1);
            else if (vals[i] == 1) Bs.Add(i, 1);

        for (int i = 0; i < vals.Count; i++)
            if (vals[i] == 1)
            {
                int R = last[who[i]];
                long inA = As.Sum(i + 1, R);
                long outA = totalA - inA;
                long inB = Bs.Sum(i + 1, R);
                long outB = totalB - 1 - inB;
                ans += inA * outB;
                ans += inB * outA;
            }

        Console.WriteLine(ans);
        Console.ReadLine();
    }

    void Rec(int a, int prev, List<int>[] G, int[] Val, List<int> who, List<int> vals, int[] last)
    {
        who.Add(a);
        vals.Add(Val[a]);
        foreach (int b in G[a])
            if (b != prev)
                Rec(b, a, G, Val, who, vals, last);
        last[a] = who.Count;
        who.Add(a);
        vals.Add(~Val[a]);
    }

    class BinaryIndexedTree
    {
        private readonly int N;
        private readonly long[] A;

        public BinaryIndexedTree(int N)
        {
            this.N = N + 1;
            A = new long[N + 2];
        }
        public void Add(int i, long val)
        {
            if (i < 0) return;
            for (++i; i <= N; i += i & -i) A[i] += val;
        }
        public void Update(int i, long val)
        {
            Add(i, val - (Sum(i) - Sum(i - 1)));
        }
        public long Sum(int i) // [0, i]
        {
            if (i < 0) return 0;
            long sum = 0;
            for (++i; i > 0; i &= i - 1) sum += A[i];
            return sum;
        }
        public long Sum(int L, int R) // [L, R)
        {
            return Sum(R - 1) - Sum(L - 1);
        }
        public int LowerBound(long val, int L = 0, int R = -1)
        {
            if (R == -1) R = N;
            L--;
            while (R - L > 1)
            {
                int mid = L + R >> 1;
                if (Sum(mid) >= val) R = mid;
                else L = mid;
            }
            return R;
        }
        public int UpperBound(long val, int L = 0, int R = -1)
        {
            if (R == -1) R = N;
            L--;
            while (R - L > 1)
            {
                int mid = L + R >> 1;
                if (Sum(mid) > val) R = mid;
                else L = mid;
            }
            return R;
        }
    }
}


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;
    }
}