#include <bits/stdc++.h>
using namespace std;
signed main(){
int n; cin >> n;
vector <vector<int>> adj(n);
for ( int i = 1; i < n; i++ ){
int u, v; cin >> u >> v;
--u, --v;
adj[u].push_back(v);
adj[v].push_back(u);
}
vector <int> a(n);
for ( auto &u: a ) cin >> u;
int opt = n + 1;
for ( int mask = 0; mask < (1 << n); mask++ ){
vector <int> b = a;
for ( int i = 0; i < n; i++ ){
if ( mask >> i & 1 ){
b[i] ^= 1;
for ( auto &v: adj[i] ) b[v] ^= 1;
}
}
int mx = 0;
for ( auto &v: b ) mx = max(mx, v);
if ( mx == 0 ){
int sz = __builtin_popcount(mask);
//~ if ( opt != n + 1 ) assert(false);
opt = min(opt, sz);
}
}
if ( opt == n + 1 ) return cout << "impossible\n", 0;
cout << opt << '\n';
}
