#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
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);
}
i64 x = 0;
for ( int i = 1; i <= n; i++ ){
int b; cin >> b;
x = x * 2 + b;
}
int k = n / 2;
map <i64,int> mp; mp[0] = 0;
for ( int mask = 0; mask < (1 << k); mask++ ){
i64 y = 0;
for ( int i = 0; i < k; i++ ){
if ( mask >> i & 1 ){
y ^= 1LL << i;
for ( auto &v: adj[i] ) y ^= 1LL << v;
}
}
int s = __builtin_popcount(mask);
if ( !mp.count(y) || mp[y] > s ) mp[y] = s;
}
int opt = n + 1;
for ( int mask = 0; mask < (1 << (n - k)); mask++ ){
i64 y = 0;
for ( int i = 0; i + k < n; i++ ){
if ( mask >> i & 1 ){
y ^= 1LL << (i + k);
for ( auto &v: adj[i + k] ) y ^= 1LL << v;
}
}
int s = __builtin_popcount(mask);
if ( mp.count(y ^ x) ) opt = min(opt, mp[y ^ x] + s);
}
if ( opt == n + 1 ) return cout << "impossible\n", 0;
cout << opt << '\n';
}
