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

int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  ll N;
  cin >> N;

  vector<vector<ll>> g(N);
  for(ll i = 0, a, b; i < N - 1; i++) {
    cin >> a >> b;
    a--, b--;
    g[a].emplace_back(b);
    g[b].emplace_back(a);
  }

  vector<ll> C(N);
  for(auto &i : C) { cin >> i; }

  vector<ll> p(N, -1);
  ll ans = 0;
  auto DFS = [&](auto &&DFS, ll v) -> bool {
    for(auto &nv : g[v]) {
      if(nv == p[v]) { continue; }
      p[nv] = v;
      C[v] ^= DFS(DFS, nv);
    }
    ans += !C[v];
    return !C[v];
  };

  cout << (DFS(DFS, 0) ? -1 : ans) << "\n";
}