#include <bits/stdc++.h>
//#include <atcoder/all>
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
//const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }
vector<int>to[200005];
vector<int>c;
P dfs(int v, int d = 0, int p = -1) {
	P res = {0,0};
	for (int e : to[v]) {
		if (p == e) continue;
		auto get = dfs(e, d + 1, v);
		res.first += get.first;
		c[v] ^= get.second;
	}
	if (!c[v]) {
		res.first++;
		res.second++;
	}
	return res;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n; cin >> n;
	rep(i, n - 1) {
		int a, b; cin >> a >> b; a--; b--;
		to[a].push_back(b);
		to[b].push_back(a);
	}
	c.resize(n);
	rep(i, n)cin >> c[i];
	auto get = dfs(0);
	if (get.second)cout << -1 << endl;
	else cout << get.first << endl;
	return 0;
}