#include <bits/stdc++.h>
using namespace std;
int n;
vector<vector<int>> adj;
vector<int> cameras;
int main() {
cin >> n;
adj.resize(n);
for (int i = 0; i < n - 1; i++) {
int a, b;
cin >> a >> b;
a--;
b--;
adj[a].push_back(b);
adj[b].push_back(a);
}
cameras.resize(n);
int def = 0;
for (int i = 0; i < n; i++) {
int a;
cin >> a;
cameras[i] = a;
if (a)
def |= 1 << i;
}
int ans = 1e9;
for (int mask = 0; mask < pow(2, n); mask++) {
int curr = def;
int count = 0;
for (int i = 0; i < n; i++) {
if (1 & (mask >> i)) {
count++;
curr ^= 1 << i;
for (int u : adj[i])
curr ^= 1 << u;
}
}
if (curr == 0) {
ans = min(ans, count);
}
}
if (ans == 1e9)
cout << "impossible" << endl;
else
cout << ans << endl;
return 0;
}
