#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) {
vector<int> child;
for (int v : adj[u]) {
if (v != par) {
dfs(v, u);
child.push_back(v);
}
}
int m = child.size();
vector<array<array<int, 2>, 2>> f(m + 1, {{{inf, inf}, {inf, inf}}});
f[0][0][a[u]] = 0;
f[0][1][a[u] ^ 1] = 0;
for (int i = 1; i <= m; i++) {
int v = child[i - 1];
for (int p = 0; p < 2; p++) {
for (int t = 0; t < 2; t++) {
if (f[i - 1][p][t] >= inf) continue;
f[i][p][t] = min(f[i][p][t], f[i - 1][p][t] + dp[v][0][0]);
f[i][p][t] = min(f[i][p][t], f[i - 1][p][t] + dp[v][1][0]);
f[i][p][t ^ 1] = min(f[i][p][t ^ 1], f[i - 1][p][t] + dp[v][0][1]);
f[i][p][t ^ 1] = min(f[i][p][t ^ 1], f[i - 1][p][t] + dp[v][1][1]);
}
}
}
dp[u][0][0] = f[m][0][0];
dp[u][0][1] = f[m][0][1];
dp[u][1][0] = f[m][1][0] + 1;
dp[u][1][1] = f[m][1][1] + 1;
}
int main() {
ios::sync_with_stdio(false);
cin.tie(nullptr);
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, 0);
int res = min(dp[1][0][0], dp[1][1][0]);
cout << (res >= inf ? "impossible" : to_string(res)) << '\n';
return 0;
}
