#include <bits/stdc++.h> #define ll long long using namespace std; int main() { ios_base::sync_with_stdio(0); cin.tie(0); ll n; cin >> n; vector<vector<ll>> adj(n); for (int i=0; i<n-1; i++){ ll u,v; cin >> u >> v; u--; v--; adj[u].push_back(v); adj[v].push_back(u); } vector<ll> a(n); for (int i=0; i<n; i++){ cin >> a[i]; } ll res = INT_MAX; for (ll mask = 0; mask<(1LL<<n); mask++){ vector<ll> b = a; ll k = 0; for (int i=0; i<n; i++){ if ((mask>>i) & 1){ k++; b[i] = 1-b[i]; for (auto v : adj[i]) { b[v] = 1-b[v]; } } } ll da = 1; for (int i=0; i<n; i++){ if (b[i]==1) { da = 0; break; } } if (da) { res = min(res, k); } } if (res==INT_MAX){ cout << "impossible" << endl; } else { cout << res << endl; } }