#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define ld long double
#define fi first
#define se second
vector<int> adj[100005];
int a[100005];
int main() {
ios_base::sync_with_stdio(false); cin.tie(NULL);
int n;
cin >> n;
for (int i = 0; i < n - 1; 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];
if (n <= 20) {
for (int i = 1; i <= n; i++) adj[i].push_back(i);
int ans = n + 1;
for (int i = 0; i < (1 << n); i++) {
int tm = 0;
for (int j = 0; j < n; j++) {
if ((1 << j) & i) for (auto x : adj[j + 1]) tm ^= (1 << (x - 1));
}
for (int j = 0; j < n; j++) if (a[j + 1]) tm ^= (1 << j);
if (tm != 0) continue;
int cnt = 0;
for (int j = 0; j < n; j++) if ((1 << j) & i) cnt++;
ans = min(ans, cnt);
}
if (ans == n + 1) cout << "impossible\n";
else cout << ans << "\n";
}
}
