#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 100'000 + 10;
int n, dp[MAX_N][2][2], ans;
vector<int> adj[MAX_N];
bitset<MAX_N> c;
void dfs(int u, int p) {
dp[u][0][c[u]] = 0;
dp[u][0][!c[u]] = MAX_N;
dp[u][1][!c[u]] = 1;
dp[u][1][c[u]] = MAX_N;
for (int v : adj[u]) {
if (v == p) continue;
dfs(v, u);
int upd[2][2] = {{MAX_N, MAX_N}, {MAX_N, MAX_N}};
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j) {
for (int k = 0; k < 2; ++k) {
upd[i][j ^ k] = min(upd[i][j ^ k], dp[u][i][j] + dp[v][k][i]);
}
}
}
swap(dp[u], upd);
}
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr), cout.tie(nullptr);
cin >> n;
for (int e = 1, u, v; e <= n - 1; ++e) {
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int u = 1; u <= n; ++u) {
int x;
cin >> x;
c[u] = x;
}
dfs(1, -1);
ans = min(dp[1][0][0], dp[1][1][0]);
if (ans == MAX_N) {
cout << "impossible\n";
} else {
cout << ans << '\n';
}
}
