#include <bits/stdc++.h>
using namespace std;
const int N = 100000;
vector<int> g[N];
int dp[N][2][2], a[N];
void DFS(int u = 0, int p = -1) {
for (int v : g[u]) {
if (v != p) {
DFS(v, u);
}
}
for (int par = 0; par < 2; par++) {
for (int cur = 0; cur < 2; cur++) {
int ans[2] = {N + 1, N + 1};
ans[par ^ cur] = cur;
for (int v : g[u]) {
if (v == p) {
continue;
}
int new_ans[2] = {N + 1, N + 1};
for (int c = 0; c < 2; c++) {
for (int d = 0; d < 2; d++) {
new_ans[d ^ c] = min(new_ans[d ^ c], ans[d] + dp[v][cur][c]);
}
}
ans[0] = new_ans[0];
ans[1] = new_ans[1];
}
dp[u][par][cur] = ans[a[u]];
}
}
}
void solve() {
int n;
cin >> n;
for (int i = 0; i < n - 1; i++) {
int u, v;
cin >> u >> v;
u--, v--;
g[u].push_back(v);
g[v].push_back(u);
}
for (int i = 0; i < n; i++) {
cin >> a[i];
}
DFS();
int ans = min(dp[0][0][0], dp[0][0][1]);
if (ans == N + 1) {
cout << "impossible\n";
} else {
cout << ans << "\n";
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}
