#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
int n, a[100005];
vector <int> adj[100005];
int dp[100005][2][2];
void dfs(int u, int par) {
for (auto v : adj[u]) {
if (v != par) {
dfs(v, u);
}
}
vector <int> child;
child.push_back(-1);
for (auto v : adj[u]) {
if (v != par) {
child.push_back(v);
}
}
int f[child.size()][2][2]; // xet den con child[i], switch hay khong, chan hay le(0 chan 1 le)
for (int p = 0; p < 2; p++) for (int j = 0; j < 2; j++) f[0][p][j] = inf;
f[0][0][a[u]] = 0, f[0][0][a[u] ^ 1] = 0;
for (int i = 1; i < child.size(); i++) {
for (int j = 0; j <= 1; j++) {
f[i][0][j] = min(f[i - 1][0][j ^ 1] + dp[child[i]][1][0], f[i - 1][0][j] + dp[child[i]][0][0]);
f[i][1][j] = min(f[i - 1][1][j ^ 1] + dp[child[i]][1][1], f[i - 1][1][j] + dp[child[i]][0][1]);
}
}
int x = child.size() - 1;
dp[u][0][a[u]] = f[x][0][0];
dp[u][1][a[u]] = f[x][1][1] + 1;
dp[u][0][a[u] ^ 1] = f[x][1][0];
dp[u][1][a[u] ^ 1] = f[x][0][1] + 1;
}
/*
dp[u][0][a[i]] -> ko tac dong len con u
chan con dp[v][1][0] + con lai dp[v][0][0]
dp[u][0][a[i] ^ 1] -> ko tac dong len con u
le con dp[v][1][0] + con lai dp[v][0][0]
dp[u][1][a[i]] -> tac dong len con u
le con dp[v][1][1] + con lai dp[v][0][1]
dp[u][1][a[i] ^ 1] -> tac dong len con u
chan con dp[v][1][1] + con lai dp[v][0][1]
*/
signed main() {
cin >> n;
for (int i = 1; i < n; i++) {
int u, v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int i = 1; i <= n; i++) cin >> a[i];
dfs(1, 1);
int x = min(dp[1][1][0], dp[1][0][0]);
if (x >= inf) cout << "impossible";
else cout << x;
return 0;
}
