import std.stdio;
import std.array;
import std.string;
import std.conv;
import std.algorithm;
import std.typecons;
import std.range;

string S;
int[][] rinsetsu;

long dfs(char c, int n, int prev) {
  long ret = 0;
  foreach (nn; rinsetsu[n])
    if (nn != prev)
      ret += dfs(c, nn, n);
  return S[n] == c ? ret+1 : ret;
}

void main() {
  int N = readln().chomp.to!int;
  S = readln().chomp;
  rinsetsu = new int[][](N);
  foreach (i; iota(N-1)) {
    auto input = readln().split.map!(to!int);
    rinsetsu[input[0]-1] ~= input[1]-1;
    rinsetsu[input[1]-1] ~= input[0]-1;
  }

  long ans = 0;
  foreach (root; iota(N)) {
    if (S[root] != 'w')
      continue;
    int len = rinsetsu[root].length.to!int;
    long[] c = new long[](len); 
    long[] w = new long[](len);
    foreach (i; iota(len)) {
      c[i] = dfs('c', rinsetsu[root][i], root);
      w[i] = dfs('w', rinsetsu[root][i], root);
    }
    foreach (i; iota(len))
      foreach (j; iota(i+1, len))
        ans += c[i]*w[j] + c[j]*w[i];
  }
  writeln(ans);
}