// とりあえず TLE 解

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const ll mod = 1e9 + 7;

int N;
string s;
vector<vector<int>> G;

// first: ww second: w
pll dfs(int v, int p) {
    pll ret;
    if (s[v] == 'w') ret.second = 1;
    for (int ch : G[v]) if (ch != p) {
        pll tmp = dfs(ch, v);
        ret.first += tmp.first;
        ret.second += tmp.second;
        if (s[v] == 'w') ret.first += tmp.second;
    }
    return ret;
}

signed main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(0);

    cin >> N;
    cin >> s;
    for (int i = 0; i < N; i++) assert(s[i] == 'c' || s[i] == 'w');

    G.resize(N);
    for (int i = 0; i < N-1; i++) {
        int a, b;
        cin >> a >> b;
        a--; b--;
        G[a].emplace_back(b);
        G[b].emplace_back(a);
    }
    // すべての c から木DP
    ll ans = 0;
    for (int i = 0; i < N; i++) {
        if (s[i] == 'c') {
            pll tmp = dfs(i, -1);
            ans += tmp.first;
        }
    }
    cout << ans << endl;
}