#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic warning "-Wunused-function"
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  VVI to(n);
  rep(_, n - 1) {
    int a, b;
    cin >> a >> b;
    a--, b--;
    to[a].emplace_back(b);
    to[b].emplace_back(a);
  }
  VI c(n);
  rep(i, n) cin >> c[i];
  if ((n - accumulate(all(c), 0)) % 2) {
    cout << -1 << '\n';
    return 0;
  }
  int ans = 0;
  auto dfs = [&](auto self, int u, int p) -> bool {
    bool now = false;
    for(int v: to[u]) if (v != p) now ^= self(self, v, u);
    c[u] ^= now;
    ans += !c[u];
    return !c[u];
  };
  if (dfs(dfs, 0, -1)) assert(false);
  cout << ans << '\n';
}