using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.IO;

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

    Scanner cin;

    int N;
    string S;
    List<int>[] es;
    long ans;
    int AllC, AllW;

    void calc()
    {
        cin = new Scanner();
        N = cin.nextInt();
        S = cin.next();
        AllC = AllW = 0;
        foreach (var c in S)
        {
            if (c == 'c') AllC++;
            else AllW++;
        }

        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);
        }
        ans = 0;
        dfs(0, -1);
        Console.WriteLine(ans);
    }

    Tuple<int, int> dfs(int p, int pre)
    {
        List<Tuple<int, int>> lt = new List<Tuple<int, int>>();
        foreach (var to in es[p])
        {
            if (to == pre) continue;
            lt.Add(dfs(to, p));
        }

        int sumC = 0;
        int sumW = 0;
        foreach (var item in lt)
        {
            sumC += item.Item1;
            sumW += item.Item2;
        }
        if (S[p] == 'c') sumC++;
        else sumW++;

        if(p != 0)
        {
            lt.Add(Tuple.Create(AllC - sumC, AllW - sumW));
        }

        long addAns = 0;
        if (S[p] == 'w')
        {
            AllW--;

            foreach (var item in lt)
            {
                addAns += (AllC - (long)item.Item1) * item.Item2;
            }

            AllW++;
        }
        ans += addAns;
        //Console.WriteLine(p + " " + addAns + " " + sumC + " " + sumW);

        return Tuple.Create(sumC, sumW);
    }
}


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);
        i = 0;
        return next();
    }

    public int nextInt()
    {
        return int.Parse(next());
    }

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

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

}