#include <bits/stdc++.h>
using namespace std;
#define int long long
#define endl '\n';
const int M = 1e5+5, MOD = 1e9+7;
vector<int> node[M];
int c[M];
signed main() {
cin.tie(0)->sync_with_stdio(0);
int n;
cin >> n;
for (int i = 1; i < n; i++) {
int a, b;
cin >> a >> b;
node[a].push_back(b);
node[b].push_back(a);
}
for (int i = 1; i <= n; i++) cin >> c[i];
map<int, int> fr, cum;
int ans = n+1, pp = 0;
for (int i = 1; i <= n; i++) pp += c[i]*(1<<i)/2;
for (int m = 1; m < (1<<(n/2)); m++) {
int x = 0;
for (int i = 1; i <= n/2; i++) {
if (m&(1<<(i-1))) {
for (int k:node[i]) x += (1<<(k-1));
x += (1<<(i-1));
}
}
if (x == pp) ans = min(ans, (int)__builtin_popcount(m));
if (!fr[x]) fr[x] = __builtin_popcount(m), cum[x] = m;
else fr[x] = min(fr[x], (int)__builtin_popcount(m));
}
for (int m = 1; m < (1<<(n-n/2)); m++) {
int x = 0;
for (int i = 0; i < n; i++) x += c[i+1]*(1<<i);
for (int i = n/2+1; i <= n; i++) {
if (m&(1<<(i-n/2-1))) {
for (int j:node[i]) x ^= (1<<(j-1));
x ^= (1<<(i-1));
}
}
if (fr[x]) ans = min(ans, fr[x]+__builtin_popcount(m));
if (!x) ans = min(ans, (int)__builtin_popcount(m));
}
cout << (ans>n?"impossible":to_string(ans)) << endl;
return 0;
}
/*
5
1 2
1 3
2 4
2 5
0 1 0 1 1
5
1 2
2 3
3 4
4 5
0 1 1 1 1
*/
