#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
ll ILL=2167167167167167167;
const int INF=2100000000;
const int mod=998244353;
#define rep(i,a,b) for (ll i=a;i<b;i++)
#define all(p) p.begin(),p.end()
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";}
template<class T> void vec_out(vector<T> &p){for(int i=0;i<(int)(p.size());i++){if(i) cout<<" ";cout<<p[i];}cout<<"\n";}
template<class T> T min(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmin(ans,x);return ans;}
template<class T> T max(vector<T> &a){assert(!a.empty());T ans=a[0];for(auto &x:a) chmax(ans,x);return ans;}
template<class T> T sum(vector<T> &a){assert(!a.empty());T ans=a[0]-a[0];for(auto &x:a) ans+=x;return ans;}

std::vector<std::vector<int>> tree_in(int N){
	std::vector<std::vector<int>> G(N);
	for(int i=0;i<N-1;i++){
		int a;int b;
		cin>>a>>b; 
		a--,b--;
		G[a].push_back(b);
		G[b].push_back(a);
	}
	return G;
}
std::tuple<std::vector<int>,std::vector<int>,std::vector<int>> tree_order_pare_depth(std::vector<std::vector<int>> &G,int root){
	int n=G.size();
	std::vector<int> order={root},pare(n,-1),depth(n);
	pare[root]=-2;
	for(int i=0;i<n;i++){
		int a=order[i];
		for(auto x:G[a]){
			if(pare[x]==-1){
				pare[x]=a;
				depth[x]=depth[a]+1;
				order.push_back(x);
			}
		}
	}
	return {order,pare,depth};
}
std::vector<int> tree_diameter_path(std::vector<std::vector<int>> &G){
	int n=G.size();
	auto r=(std::get<0>(tree_order_pare_depth(G,0))).at(n-1);
	std::vector<int> order,pare,depth,ans;
	tie(order,pare,depth)=tree_order_pare_depth(G,r);
	int ind=order[n-1];
	while(ind!=-2){
		ans.push_back(ind);
		ind=pare[ind];
	}
	return ans;
}


void solve();
// oddloop
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int t=1;
	//cin>>t;
	rep(i,0,t) solve();
}

void solve(){
	int N;
	cin>>N;
	auto G=tree_in(N);
	vector<int> pare,order,dep;
	tie(order,pare,dep)=tree_order_pare_depth(G,0);
	vector<int> p(N);
	rep(i,0,N){
		cin>>p[i];
	}
	int ans=0;
	for(int i=N-1;i>0;i--){
		int a=order[i];
		if(p[a]==0){
			p[pare[a]]^=1;
			ans++;
		}
	}
	cout<<(p[0] ? ans : -1)<<"\n";
}