#include <bits/stdc++.h>
#ifndef LOCAL_TEST
#pragma GCC target ("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#endif // LOCAL_TEST
using namespace std;

using ll = long long;
using pii = pair<int, int>; using pll = pair<ll, ll>;
using vi = vector<int>;  using vvi = vector<vi>; using vvvi = vector<vvi>;
using vl = vector<ll>;  using vvl = vector<vl>; using vvvl = vector<vvl>;
using vb = vector<bool>; using vvb = vector<vb>; using vvvb = vector<vvb>;
using vc = vector<char>; using vvc = vector<vc>; using vvvc = vector<vvc>;
using vd = vector<double>; using vvd = vector<vd>; using vvvd = vector<vvd>;
using vs = vector<string>; using vvs = vector<vector<string>>; using vvvs = vector<vector<vector<string>>>;
template <class T> using priority_queue_min = priority_queue<T, vector<T>, greater<T>>;

const double PI = acos(-1.0);
int INF = 100100111; ll INFL = 3300300300300300491LL;
double EPS = 1e-8; double EPSL = 1e-16;
constexpr int MODINT998244353 = 998244353;
constexpr int MODINT1000000007 = 1000000007;
struct fast_io { fast_io() { cin.tie(nullptr); ios::sync_with_stdio(false); cout << fixed << setprecision(18); } } fastIOtmp;

#define all(a) (a).begin(), (a).end()
#define sz(x) ((int)(x).size())
#define lbpos(a, x) (int)distance((a).begin(), std::lower_bound(all(a), x))
#define ubpos(a, x) (int)distance((a).begin(), std::upper_bound(all(a), x))
#define rep(i, n) for(ll i = 0, i##_len = int(n); i < i##_len; ++i) // 0 から n-1 まで昇順
#define repi(i, s, t) for(ll i = ll(s), i##_end = ll(t); i < i##_end; ++i) // s から t まで昇順
#define repis(i, s, t, step) for(ll i = ll(s), i##_end = ll(t); i < i##_end; i+=step) // s から t まで stepずつ
#define repir(i, s, t, step) for(ll i = ll(s), i##_end = ll(t); i > i##_end; i+=step) // s から t まで stepずつ
#define repe(a, v) for(auto& a : (v)) // v の全要素（変更可能）
#define repea(a, v) for(const auto& a : (v)) // v の全要素（変更不可能）
#define smod(n, m) ((((n) % (m)) + (m)) % (m)) // 非負mod
#define sdiv(n, m) (((n) - smod(n, m)) / (m)) // 非負div
#define uniq(a) {sort(all(a)); (a).erase(unique(all(a)), (a).end());} // 重複除去
#define EXIT(a) {cout << (a) << endl; exit(0);} // 強制終了
void Yes(bool b) { cout << (b ? "Yes\n" : "No\n"); return; }

ll powm(ll a, ll n, ll mod=INFL) {
    ll res = 1;
    while (n > 0) {
        if (n & 1) res = (res * a) % mod;
        a = (a * a) % mod;
        n >>= 1;
    }
    return res;
}
template <class T> inline bool chmax(T& M, const T& x) { if (M < x) { M = x; return true; } return false; } // 最大値を更新（更新されたら true を返す）
template <class T> inline bool chmin(T& m, const T& x) { if (m > x) { m = x; return true; } return false; } // 最小値を更新（更新されたら true を返す）
int digit(ll x, int d=10) { int rev=0; while (x > 0) { rev++; x /= d;}; return rev; } // xのd進数桁数

template <class Cost> struct Graph{
public:
    struct Edge {
        int to;
        Cost cost;
        Edge() {};
        Edge(int _to, Cost _cost) : to(_to), cost(_cost) {};
    };
    struct AdjacencyListRange{
        using iterator = typename std::vector<Edge>::const_iterator;
        iterator begi, endi;
        iterator begin() const { return begi; }
        iterator end() const { return endi; }
        int size() const { return (int)distance(begi, endi); }
        const Edge& operator[](int i) const { return begi[i]; }
    };
private:
public:
    vector<Edge> E;
    vector<int> I;
    int n;
    Graph() : n(0) {}
    Graph(int _n) : n(_n) {}
    Graph(int _n, const vector<int>& from, vector<int>& to, vector<Cost>& cost, bool rev = false) : n(_n) {
        vector<int> buf(n+1, 0);
        for(int i=0; i<(int)from.size(); i++){
            ++buf[from[i]];
            if (rev) ++buf[to[i]];
        }
        for(int i=1; i<=_n; i++) buf[i] += buf[i-1];
        E.resize(buf[n]);
        for(int i=(int)from.size()-1; i>=0; i--){
            int u = from[i];
            int v = to[i];
            Cost c = cost[i];
            E[--buf[u]] = Edge(v, c);
            if(rev) E[--buf[v]] = Edge(u, c);
        }
        I = move(buf);
    }
    AdjacencyListRange operator[](int u) const {
        return AdjacencyListRange{ E.begin() + I[u], E.begin() + I[u+1] };
    }
    int num_vertices() const { return n; }
    int size() const { return num_vertices(); }
    int num_edges() const { return E.size(); }
};
template <class T> ostream& operator<<(ostream& os, Graph<T> g) {
    bool first = true;
    rep(i, g.n) repe(e, g[i]) {
        if (first) first = false;
        else os << endl;
        os << i << "->" << e.to << ": " << e.cost;
    }
    return os;
}
#include <bits/stdc++.h>
using namespace std;
template <class T, class U> inline istream& operator>>(istream& is, pair<T, U>& p);
template <class T> inline istream& operator>>(istream& is, vector<T>& v);
template <class T, class U> inline ostream& operator<<(ostream& os, const pair<T, U>& p);
template <class T> inline ostream& operator<<(ostream& os, const vector<T>& v);
template <typename T, typename S> ostream &operator<<(ostream &os, const map<T, S> &mp);
template <typename T> ostream &operator<<(ostream &os, const set<T> &st);
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st);
template <typename T> ostream &operator<<(ostream &os, queue<T> q);
template <typename T> ostream &operator<<(ostream &os, deque<T> q);
template <typename T> ostream &operator<<(ostream &os, stack<T> st);
template <class T, class Container, class Compare> ostream &operator<<(ostream &os, priority_queue<T, Container, Compare> pq);

template <class T, class U> inline istream& operator>>(istream& is, pair<T, U>& p) { is >> p.first >> p.second; return is; }
template <class T> inline istream& operator>>(istream& is, vector<T>& v) { repe(x, v) is >> x; return is; }
template <class T, class U> inline ostream& operator<<(ostream& os, const pair<T, U>& p) { os << p.first << " " << p.second; return os; }
template <class T> inline ostream& operator<<(ostream& os, const vector<T>& v) { rep(i, sz(v)) { os << v.at(i); if (i != sz(v) - 1) os << " "; } return os; }
template <typename T, typename S> ostream &operator<<(ostream &os, const map<T, S> &mp) { for (auto &[key, val] : mp) { os << key << ":" << val << " "; } return os; }
template <typename T> ostream &operator<<(ostream &os, const set<T> &st) { auto itr = st.begin(); for (int i = 0; i < (int)st.size(); i++) { os << *itr << (i + 1 != (int)st.size() ? " " : ""); itr++; } return os; }
template <typename T> ostream &operator<<(ostream &os, const multiset<T> &st) { auto itr = st.begin(); for (int i = 0; i < (int)st.size(); i++) { os << *itr << (i + 1 != (int)st.size() ? " " : ""); itr++; } return os; }
template <typename T> ostream &operator<<(ostream &os, queue<T> q) { while (q.size()) { os << q.front() << " "; q.pop(); } return os; }
template <typename T> ostream &operator<<(ostream &os, deque<T> q) { while (q.size()) { os << q.front() << " "; q.pop_front(); } return os; }
template <typename T> ostream &operator<<(ostream &os, stack<T> st) { while (st.size()) { os << st.top() << " "; st.pop(); } return os; }
template <class T, class Container, class Compare> ostream &operator<<(ostream &os, priority_queue<T, Container, Compare> pq) { while (pq.size()) { os << pq.top() << " "; pq.pop(); } return os; }
template <class T> inline vector<T>& operator--(vector<T>& v) { repe(x, v) --x; return v; }
template <class T> inline vector<T>& operator++(vector<T>& v) { repe(x, v) ++x; return v; }

#define debug(...) debug_func(0, #__VA_ARGS__, __VA_ARGS__) // debug print
#ifdef LOCAL_TEST
template <typename T>
void debug_func(int i, T name) { (void)i; (void)name; cerr << endl; }
template <typename T1, typename T2, typename... T3> void debug_func(int i, const T1 &name, const T2 &a, const T3 &...b) {
  for ( ; name[i] != ',' && name[i] != '\0'; i++ ) cerr << name[i];
  cerr << ":" << a << " ";
  debug_func(i + 1, name, b...);
}
#endif
#ifndef LOCAL_TEST
template <typename... T>
void debug_func(const T &...) {}
#endif

int main() {
    int n;
    cin >> n;
    assert(2 <= n && n <= 2 * powm(10, 5));
    vi from(n-1), to(n-1), cost(n-1, 1);
    rep(i, n-1) cin >> from[i] >> to[i];
    rep(i, n-1) {
        assert(1 <= from[i] && from[i] <= n);
        assert(1 <= to[i] && to[i] <= n);
        assert(from[i] < to[i]);
    }
    --from; --to;
    Graph g(n, from, to, cost, true);
    vi c(n);
    cin >> c;
    rep(i, n) assert(c[i] == 0 || c[i] == 1);
    int ans = 0;
    vb seen(n, false);
    auto dfs = [&] (auto self, int x) -> bool {
        if (seen[x]) return false;
        seen[x] = true;
        int cur = 0;
        repe(e, g[x]) {
            if (self(self, e.to)) {cur++;}
        }
        if ((cur + c[x]) % 2) {
            debug(x, 0);
            return false;
        } else {
            debug(x, 1);
            ans++;
            return true;
        }
    };
    if (dfs(dfs, 0)) cout << -1 << endl;
    else cout << ans << endl;
    return 0;
}