#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define int long long
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORD(i, a, b) for (int i = (b); i >= (a); i --)
#define REP(i, n) for (int i = 0; i < (n); ++i)
#define REPD(i, n) for (int i = (n) - 1; i >= 0; --i)
#define MASK(i) (1LL << (i))
#define BIT(x, i) (((x) >> (i)) & 1)
constexpr ll LINF = (1ll << 60);
constexpr int INF = (1ll << 30);
constexpr int Mod = 1e9 + 7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
/*
Phu Trong from Nguyen Tat Thanh High School for gifted student
*/
template <class X, class Y>
bool minimize(X &x, const Y &y){
X eps = 1e-9;
if (x > y + eps) {x = y; return 1;}
return 0;
}
template <class X, class Y>
bool maximize(X &x, const Y &y){
X eps = 1e-9;
if (x + eps < y) {x = y; return 1;}
return 0;
}
#define MAX 200005
int numNode;
vector<int> G[MAX];
int label[MAX];
vector<int> col;
void press(int x){
col[x] ^= 1;
}
void process(void){
cin >> numNode;
if(numNode > 20) return void(cout << -1);
for (int i = 1; i < numNode; ++i){
int u, v; cin >> u >> v;
--u, --v;
G[u].emplace_back(v);
G[v].emplace_back(u);
}
for (int i = 0; i < numNode; ++i) cin >> label[i];
int ans = INF;
col.resize(numNode);
for (int msk = 0; msk < MASK(numNode); ++msk){
for (int i = 0; i < numNode; ++i) col[i] = label[i];
for (int i = 0; i < numNode; ++i) if(BIT(msk, i)){
press(i);
for (int&v : G[i]) press(v);
}
int sm = accumulate(col.begin(), col.end(), 0);
if(!sm) minimize(ans, __builtin_popcount(msk));
}
cout << (ans >= INF ? -1 : ans);
}
signed main(){
#define name "Whisper"
cin.tie(nullptr) -> sync_with_stdio(false);
//freopen(name".inp", "r", stdin);
//freopen(name".out", "w", stdout);
process();
}
