#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 1000000000000000001
vector<vector<int>> E;
int ans = 0;
vector<int> c;

void dfs(int cv,int p){
	rep(i,E[cv].size()){
		int to = E[cv][i];
		if(to==p)continue;
		dfs(to,cv);
	}
	if(c[cv]==0 && p!=-1){
		c[cv] = 1;
		c[p] ^= 1;
		ans++;
	}
}

int main(){
	
	int n;
	cin>>n;
	
	E.resize(n);
	c.resize(n);
	rep(i,n-1){
		int a,b;
		cin>>a>>b;
		a--,b--;
		E[a].push_back(b);
		E[b].push_back(a);
	}
	rep(i,n)cin>>c[i];
	dfs(0,-1);
	if(c!=vector<int>(n,1))cout<<-1<<endl;
	else cout<<ans<<endl;
	return 0;
}