#include <bits/stdc++.h>
using namespace std;
int n;
vector<vector<int>> adj;
vector<int> cameras;
pair<int, int> dfs(int u, bool target, int p = -1) {
bool is_leaf = true;
for (int v : adj[u])
if (v != p)
is_leaf = false;
if (is_leaf)
return {target ^ cameras[u], target ^ cameras[u]};
int ans = 1e9;
int ansflip = 0;
int zero_ans = 0;
int zero_me = cameras[u];
for (int v : adj[u])
if (v != p) {
auto [subans, flip] = dfs(v, 0, u);
zero_ans += subans;
zero_me += flip;
}
if (zero_me % 2 == target && zero_ans < ans) {
ans = zero_ans;
ansflip = 0;
}
int one_ans = 0;
int one_me = cameras[u];
for (int v : adj[u])
if (v != p) {
auto [subans, flip] = dfs(v, 1, u);
one_ans += subans;
one_me += flip;
}
if (one_me % 2 != target && one_ans + 1 < ans) {
ans = one_ans + 1;
ansflip = 1;
}
return {ans, ansflip};
}
int main() {
cin >> n;
adj.resize(n);
for (int i = 0; i < n - 1; i++) {
int a, b;
cin >> a >> b;
a--;
b--;
adj[a].push_back(b);
adj[b].push_back(a);
}
cameras.resize(n);
for (int i = 0; i < n; i++) {
int a;
cin >> a;
cameras[i] = a;
}
int ans = dfs(0, 0).first;
if (1e9 <= ans)
cout << "impossible" << endl;
else
cout << ans << endl;
return 0;
}
