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

#define rep(i,n) for (int i=0;i<(n);i++)
#define rep2(i,a,b) for (int i=(a);i<(b);i++)
#define rrep(i,n) for (int i=(n)-1;i>=0;i--)
#define rrep2(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define all(a) (a).begin(),(a).end()

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

const ll mod = 1e9 + 7;

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

int dfs(int v, int p) {
    int ret = (s[v] == 'w');
    for (int ch : G[v]) if (ch != p) {
        ret += dfs(ch, v);
    }
    return ret;
}

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

    cin >> N;
    cin >> s;

    G.resize(N);
    rep(i, N - 1) {
        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;
    rep(i, N) {
        if (s[i] == 'c') {
            for (int ch : G[i]) {
                ll p = dfs(ch, i);
                ans += p*(p-1)/2;
            }
        }
    }
    cout << ans << endl;
}