#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
#define sp <<" "<<
#define endl "\n"
const int MAXN = 1e5;
const int INF = 1e9+5;
int n;
vector<int> val;
vector<vector<int>> adj;
int dp[MAXN][2][2];
void dfs(int u, int p = -1) {
for (int f = 0; f < 2; f++) {
dp[u][f][val[u] ^ f] = f;
dp[u][f][val[u] ^ f ^ 1] = INF;
}
for (auto &v : adj[u]) {
if (v == p) continue;
dfs(v, u);
for (int f = 0; f < 2; f++) {
int o0 = dp[u][f][0], o1 = dp[u][f][1];
dp[u][f][0] = min({INF, o0 + dp[v][0][f], o1 + dp[v][1][f]});
dp[u][f][1] = min({INF, o0 + dp[v][1][f], o1 + dp[v][0][f]});
}
}
}
void solve() {
cin >> n;
val.resize(n);
adj.assign(n, {});
for (int i = 1; i < n; i++) {
int u, v; cin >> u >> v; u--, v--;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int i = 0; i < n; i++) {
cin >> val[i];
}
int root = 0;
dfs(root);
int ans = min(dp[root][0][0], dp[root][1][0]);
cout << (ans > n ? "impossible" : to_string(ans)) << endl;
}
signed main() {
cin.tie(0);
ios_base::sync_with_stdio(false);
int t = 1;
// cin >> t;
while (t--)
solve();
}
