/**
 *   @FileName	a.cpp
 *   @Author	kanpurin
 *   @Created	2020.06.04 15:55:57
 **/

#include "bits/stdc++.h"
using namespace std;
typedef long long ll;

using T = pair< ll, ll >;
int n;
vector< vector< int > > tree;
vector< T > dist;
ll ans;
string s;
void dfs1(int v, int p = -1) {
    T res = {0, 0};
    if (s[v] == 'c') {
        res.first++;
    } else {
        res.second++;
    }
    for (auto& u : tree[v]) {
        if (u == p) continue;
        dfs1(u, v);
        res.first += dist[u].first;
        res.second += dist[u].second;
    }
    dist[v] = res;
}
void dfs2(int v, T d_par, int p = -1) {

    if (s[v] == 'w') {

        for (auto& u : tree[v]) {
            if (u == p) {
                ans += d_par.first * (dist[v].second - 1);
            } else {
                ans += (dist[u].first) * (dist[v].second - 1 - dist[u].second + d_par.second);
            }
        }
    }

    auto f = [](T a, T b) { return make_pair(a.first + b.first, a.second + b.second); };
    T e = {0, 0};
    vector< T > lf(tree[v].size(), e), ri(tree[v].size(), e);
    for (int i = 1; i < tree[v].size(); i++) {
        if (tree[v][i - 1] == p)
            lf[i] = f(d_par, lf[i - 1]);
        else
            lf[i] = f(dist[tree[v][i - 1]], lf[i - 1]);
    }
    for (int i = tree[v].size() - 2; i >= 0; i--) {
        if (tree[v][i + 1] == p)
            ri[i] = f(d_par, ri[i + 1]);
        else
            ri[i] = f(dist[tree[v][i + 1]], ri[i + 1]);
    }
    for (int i = 0; i < tree[v].size(); i++) {
        if (tree[v][i] == p) continue;
        auto next = f(lf[i], ri[i]);

        if (s[v] == 'c') {
            next.first++;
        } else {
            next.second++;
        }
        dfs2(tree[v][i], next, v);
    }
}
int main() {
    cin >> n;
    tree.resize(n);
    dist.resize(n);
    cin >> s;
    for (int i = 0; i < n - 1; i++) {
        int u, v;
        cin >> u >> v;
        u--;
        v--;
        tree[u].push_back(v);
        tree[v].push_back(u);
    }
    dfs1(0);
    dfs2(0, {0, 0});
    cout << ans << endl;
    return 0;
}