#include<bits/stdc++.h>
using namespace std;
 
using ll=long long;
using pp=pair<int,int>;
#define sr string 
#define vc vector
#define db double
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define pb push_back
#define all(v) v.begin(),v.end()
#define pque priority_queue
#define bpc(a) __builtin_popcount(a)
int main(){
	int n;cin>>n;
	vc v(n,vc<int>(0));
	rep(i,n-1){
		int a,b;cin>>a>>b; a--;b--;
		v[a].pb(b); v[b].pb(a);
	}
	vc<int>c(n); rep(i,n)cin>>c[i];
	auto f=[&](auto f,int a,int p)->pp{
	    int res=0,now=c[a];
	    for(auto au:v[a])if(au!=p){
	    	auto [x,y]=f(f,au,a);
	    	res+=x;
	    	now^=y;
	    }
	    if(now==0)return {res+1,1};
	    else return {res,0};
	};
	auto [ans,zz]=f(f,0,-1);
	if(zz)cout<<-1;
	else cout<<ans;
}