#include <bits/stdc++.h>
using namespace std;
const int MAX = 1e5 + 5;
int N, c[MAX];
vector<int> adj[MAX];
namespace subtask1{
bool check(){
return N <= 20;
}
int msk[MAX];
void solve(){
int base = 0;
for(int i = 0; i < N; ++i){
base ^= c[i] * (1 << i);
}
for(int i = 0; i < N; ++i){
msk[i] = (1 << i);
for(auto j : adj[i]) msk[i] |= (1 << j);
}
int ans = N + 1;
for(int mask = 0; mask < (1 << N); ++mask){
int cur = base;
for(int i = 0; i < N; ++i){
if(mask >> i & 1) cur ^= msk[i];
}
if(cur == 0) {
ans = min(ans, __builtin_popcount(mask));
}
}
if(ans == N + 1) cout << "impossible\n";
else cout << ans << '\n';
}
}
namespace subtask2{
bool check(){
return N <= 40;
}
long long msk[MAX];
vector<pair<long long, int>> compress(const vector<pair<long long, int>>& x){
vector<pair<long long, int>> y;
for(int i = 0; i < (int)x.size(); ++i){
if(i == 0 || x[i - 1].first != x[i].first) y.push_back(x[i]);
}
return y;
}
void solve(){
long long base = 0;
for(int i = 0; i < N; ++i){
if(c[i]) base ^= (1LL << i);
}
for(int i = 0; i < N; ++i){
msk[i] = 1LL << i;
for(int j : adj[i]) msk[i] ^= (1LL << j);
}
int sz_left = N / 2, sz_right = N - sz_left;
vector<pair<long long, int>> l, r;
for(int mask = 0; mask < (1 << sz_left); ++mask){
long long cur = 0;
for(int i = 0; i < sz_left; ++i){
if(mask >> i & 1) cur ^= msk[i];
}
l.emplace_back(cur, __builtin_popcount(mask));
}
for(int mask = 0; mask < (1 << sz_right); ++mask){
long long cur = 0;
for(int i = 0; i < sz_right; ++i){
if(mask >> i & 1) cur ^= msk[sz_left + i];
}
r.emplace_back(cur, __builtin_popcount(mask));
}
sort(l.begin(), l.end());
sort(r.begin(), r.end());
vector<pair<long long, int>> candidates = compress(l);
int ans = N + 1;
long long lst = -1;
for(auto [mask, cost] : r){
if(lst == mask) continue;
lst = mask;
mask ^= base;
int l = 0, r = (int)candidates.size() - 1, pos = -1;
while(l <= r){
int mid = l + r >> 1;
if(candidates[mid].first == mask){
pos = mid;
break;
} else if(candidates[mid].first < mask) {
l = mid + 1;
} else r = mid - 1;
}
if(pos != -1){
ans = min(ans, cost + candidates[pos].second);
}
}
if(ans == N + 1) cout << "impossible\n";
else cout << ans << '\n';
}
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef LOCAL
freopen("task.inp", "r", stdin);
freopen("task.out", "w", stdout);
#endif // LOCAL
cin >> N;
for(int i = 1; i < N; ++i){
int u, v;
cin >> u >> v;
--u, --v;
adj[u].emplace_back(v); adj[v].emplace_back(u);
}
for(int i = 0; i < N; ++i) cin >> c[i];
if(subtask1::check()) return subtask1::solve(), 0;
if(subtask2::check()) return subtask2::solve(), 0;
return 0;
}
