#include <bits/stdc++.h>
using namespace std;
typedef vector<int> vi;
typedef vector<char> vc;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef vector<ll> vll;
#define sep ' '
#define F first
#define S second
#define fastIO ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define pb push_back
#define kill(x) {cout << x << endl;return;}
#define sz(x) int(x.size())
#define SP(x) setprecision(x)
#define mod(x) (1ll*x%M+M)%M
#define pq priority_queue
#define mid (l+r)/2
// #define mid2 (l+r+1)/2
#define pll pair<ll, ll>
#define REP(i, k) for (int i = 0 ; i < k ; i ++)
#define MP make_pair
#define us unordered_set
const int INF = 1e9+10;
const int N = 1e5+10;
int n;
vi adj[N];
bool s[N];
int dp[2][2][N];
void dfs(int v = 0, int p = -1) {
// cout << v+1 << endl;
if (adj[v].size() == 1 && adj[v][0] == p) {
if (s[v]) {
dp[1][0][v] = 1;
dp[0][1][v] = 0;
} else {
dp[0][0][v] = 0;
dp[1][1][v] = 1;
}
return;
}
bool flag0 = true, flag1 = true;
for(int neigh: adj[v]) {
if (neigh == p) continue;
dfs(neigh, v);
if (dp[0][0][neigh] == -1 && dp[1][0][neigh] == -1) {
flag0 = false;
}
if (dp[0][1][neigh] == -1 && dp[1][1][neigh] == -1) {
flag1 = false;
}
}
if (flag0) {
bool curr = s[v];
vector<int> vec;
int cost = 0;
for(int neigh: adj[v]) {
if (neigh == p) continue;
if (dp[0][0][neigh] == -1) {
cost += dp[1][0][neigh];
curr = !curr;
} else {
cost += dp[0][0][neigh];
if (dp[1][0][neigh] != -1) {
vec.pb(dp[1][0][neigh] - dp[0][0][neigh]);
}
}
}
dp[0][curr][v] = (dp[0][curr][v] == -1 ? cost : min(dp[0][curr][v], cost));
sort(vec.begin(), vec.end());
for(int delta: vec) {
cost += delta;
curr = !curr;
dp[0][curr][v] = (dp[0][curr][v] == -1 ? cost : min(dp[0][curr][v], cost));
}
}
if (flag1) {
bool curr = !s[v];
vector<int> vec;
int cost = 1; // we changed the v itself !
for(int neigh: adj[v]) {
if (neigh == p) continue;
if (dp[0][1][neigh] == -1) {
cost += dp[1][1][neigh];
curr = !curr;
} else {
cost += dp[0][1][neigh];
if (dp[1][1][neigh] != -1) {
vec.pb(dp[1][1][neigh] - dp[0][1][neigh]);
}
}
}
dp[1][curr][v] = (dp[1][curr][v] == -1 ? cost : min(dp[1][curr][v], cost));
sort(vec.begin(), vec.end());
for(int delta: vec) {
cost += delta;
curr = !curr;
dp[1][curr][v] = (dp[1][curr][v] == -1 ? cost : min(dp[1][curr][v], cost));
}
}
}
void solve() {
memset(dp, -1, sizeof dp);
cin >> n;
for(int i = 0 ; i < n-1 ; i ++) {
int u, v; cin >> u >> v; u--, v--;
adj[u].pb(v), adj[v].pb(u);
}
REP(i, n) cin >> s[i];
dfs();
int ans = n+1;
if (dp[0][0][0] != -1) ans = min(ans, dp[0][0][0]);
if (dp[1][0][0] != -1) ans = min(ans, dp[1][0][0]);
if (ans <= n) cout << ans << endl;
else cout << "impossible" << endl;
}
// check MAXN
int32_t main() {
fastIO;
solve();
return 0;
}
