#include <bits/stdc++.h>
using namespace std;
#define int long long
#define INF (int)1e18
mt19937_64 RNG(chrono::steady_clock::now().time_since_epoch().count());
const int N = 1e5 + 69;
int dp[N][2][2], a[N], n, ndp[2][2];
vector <int> adj[N];
void dfs(int u, int par = -1){
//second index --> is subtree head off
//third index --> did you use operation at subtree head
dp[u][a[u]][0] = 0;
dp[u][a[u] ^ 1][1] = 1;
for (int v : adj[u]){
if (v != par){
dfs(v, u);
ndp[0][0] = ndp[0][1] = ndp[1][0] = ndp[1][1] = INF;
for (int s = 0; s < 2; s++){
//current state of head
for (int op = 0; op < 2; op++){
//used op at current head or not
for (int op1 = 0; op1 < 2; op1++){
//used op at child or not
int need = op;
ndp[s ^ op1][op] = min(ndp[s ^ op1][op], dp[u][s][op] + dp[v][need][op1]);
}
}
}
for (int i = 0; i < 2; i++){
for (int j = 0; j < 2; j++){
dp[u][i][j] = ndp[i][j];
}
}
}
}
// cout << u << "\n";
// for (int i = 0; i < 2; i++){
// for (int j = 0; j < 2; j++){
// cout << dp[u][i][j] << " ";
// }
// }
// cout << "\n";
}
void Solve()
{
cin >> n;
for (int i = 1; i < n; i++) {
int u, v; cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int i = 1; i <= n; i++) cin >> a[i];
for (int i = 0; i < N; i++){
for (int j = 0; j < 2; j++){
for (int k = 0; k < 2; k++){
dp[i][j][k] = INF;
}
}
}
dfs(1);
int ans = INF;
for (int i = 0; i < 1; i++){
for (int j = 0; j < 2; j++){
ans = min(ans, dp[1][i][j]);
}
}
if (ans == INF) cout << "impossible\n";
else cout << ans << "\n";
}
int32_t main()
{
auto begin = std::chrono::high_resolution_clock::now();
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
// cin >> t;
for(int i = 1; i <= t; i++)
{
//cout << "Case #" << i << ": ";
Solve();
}
auto end = std::chrono::high_resolution_clock::now();
auto elapsed = std::chrono::duration_cast<std::chrono::nanoseconds>(end - begin);
cerr << "Time measured: " << elapsed.count() * 1e-9 << " seconds.\n";
return 0;
}
