#include <bits/stdc++.h>
#define int long long
using namespace std;
 
void file_IO(){
    freopen("cww.in", "r", stdin);
    freopen("cww.out", "w", stdout);
}
 
const int N = 1e5 + 10;
int n, cnt = 0, ans = 0, ch[N], sz[N][2];
vector<int> g[N];
 
void dfs(int u, int fno){
    sz[u][ch[u] ^ 1] = 0, sz[u][ch[u]] = 1;
    for (auto v : g[u])
        if (v ^ fno){
            dfs(v, u);
            for (int x : {0, 1})    sz[u][x] += sz[v][x];
            if (ch[u])  ans += sz[v][0] * (cnt - 1 - sz[v][1]);
        }
    if (ch[u])  ans += (n - cnt - sz[u][0]) * (sz[u][1] - 1);
}
 
signed main(){
    //file_IO();
    ios::sync_with_stdio(0);
    cin.tie(0), cout.tie(0);
    cin >> n;
    for (int i = 1; i <= n; i++){
        char x; cin >> x;
        ch[i] = (x == 'w' ? 1 : 0), cnt += ch[i];
    }
    for (int i = 1, u, v; i < n; i++){
        cin >> u >> v;
        g[u].push_back(v), g[v].push_back(u);
    }
    dfs(1, 0);
    cout << ans << endl;
    return 0;
}