#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define all(x) x.begin(),x.end()
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define rep_r(i,a,b) for(int i=a;i>=b;i--)
#define each(a,x) for (auto& x : a)
using pi = pair<int,int>;
using pl = pair<ll,ll>;
using vi = vector<int>;
using vl = vector<ll>;
#define sz(x) int(x.size())
#define so(x) sort(all(x))
#define so_r(x) sort(all(x),greater<int>())
#define lb lower_bound
#define ub upper_bound
const char nl = '\n';
int dx[4] = {1,-1,0,0};
int dy[4] = {0,0,1,-1};
int bit_cnt(int x){
    return __builtin_popcount(x);
}
ll bex(ll a, ll b, ll mod = 1e9 + 7){ll res = 1LL; while(b){ if (b&1) res = res * a % mod; a = a * a % mod; b >>= 1;} return res;}
template<class t,class u> bool chmax(t&a,u b){if(a<b){a=b;return true;}else return false;}
template<class t,class u> bool chmin(t&a,u b){if(b<a){a=b;return true;}else return false;}

const int MOD = 998244353;
const int N = 5e5 + 5;
const ll INF = 1e16;
int n; 
int ans = 0; 
vi g[N]; 
int c[N];
void dfs(int u, int fa){
    int white = 0;
    each(g[u],v){
        if (v==fa)continue;
        dfs(v,u);
        white += (c[v] == 0);
    }
    ans += white;
    if (white&1) c[u] = 1 - c[u];
}
void solve(){
    cin >> n;
    rep(i,1,n-1){
        int a,b;
        cin >> a >> b; 
        g[a].push_back(b); 
        g[b].push_back(a);
    }
    rep(i,1,n) cin >> c[i]; 
    dfs(1,1);
    if (c[1] == 0) cout << -1 << nl;
    else cout << ans << nl;
}

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);

    int t = 1; 
    // cin >> t;

    while (t--){
        solve();
    }
}