#include <bits/stdc++.h>
using namespace std;
// #undef _GLIBCXX_DEBUG // disable run-time bound checking, etc
// #pragma GCC optimize("Ofast,inline") // Ofast = O3,fast-math,allow-store-data-races,no-protect-parens
// #pragma GCC optimize ("unroll-loops")
// #pragma GCC target("bmi,bmi2,lzcnt,popcnt") // bit manipulation
// #pragma GCC target("movbe") // byte swap
// #pragma GCC target("aes,pclmul,rdrnd") // encryption
// #pragma GCC target("avx,avx2,f16c,fma,sse3,ssse3,sse4.1,sse4.2") // SIMD
// #include <bits/extc++.h>
// using namespace __gnu_pbds;
// template<class T>using ordered_set = tree<T, null_type, less<T>, rb_tree_tag,tree_order_statistics_node_update>;
// template<class T>using ordered_multiset = tree<T, null_type, less_equal<T>, rb_tree_tag,tree_order_statistics_node_update>;
#define ll long long
#define INF (ll)1e9+7
#define fo(i, n) for(int i=0;i<((ll)n);i++)
#define deb(x) cout << #x << " = " << x << endl;
#define deb2(x, y) cout << #x << " = " << x << ", " << #y << " = " << y << endl
#define pb push_back
#define mp make_pair
#define F first
#define S second
#define LSOne(S) ((S) & (-S))
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
inline int readint(){ int v = 0; char c; while((c = getchar()) != EOF && c != ' ' && c != '\n'){ v *= 10; v += c - '0'; } return v; }
inline int readintsigned() { int v = 0; int sign = 1; char c = getchar(); if (c == '-') { sign = -1; } else { v += c - '0'; } while ((c = getchar()) != EOF && c != ' ' && c != '\n') { v *= 10; v += c - '0'; } return v * sign; }
inline string readstring() { string s; char c; while ((c = getchar()) != EOF && c != '\n' && c != ' ') { s.push_back(c); } return s; }
template <class result_t=std::chrono::milliseconds,class clock_t=std::chrono::steady_clock,class duration_t = std::chrono::milliseconds>
auto since(std::chrono::time_point<clock_t, duration_t> const& start){return std::chrono::duration_cast<result_t>(clock_t::now() - start);}
typedef pair<int, int> pii;
typedef pair<ll, ll> pl;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpii;
typedef vector<pl> vpl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;
typedef vector<vpii> vvpii;
typedef vector<vpl> vvpl;
vvi adj(100001);
vi cameraOn(100001);
ll memo[100001][2][2];
ll dp(int u, int last, bool turnOff){
ll &ans = memo[u][turnOff][cameraOn[u]];
if(ans != -1) return ans;
ans = turnOff;
if(turnOff) cameraOn[u] = !cameraOn[u];
vl stored;
for(auto v : adj[u]){
if(v == last) continue;
if(turnOff) cameraOn[v] = !cameraOn[v];
ll res = dp(v, u, 0);
ans += res;
stored.pb(dp(v, u, 1)-res);
if(turnOff) cameraOn[v] = !cameraOn[v];
}
sort(all(stored));
if(cameraOn[u] && !stored.size()) {
if(turnOff) cameraOn[u] = !cameraOn[u];
return ans = INF;
}
if(cameraOn[u]) ans += stored[0];
for(int i = cameraOn[u]+1; i<stored.size(); i+=2){
ll temp = stored[i]+stored[i-1];
if(temp>=0) break;
ans += temp;
}
if(turnOff) cameraOn[u] = !cameraOn[u];
return ans;
}
int main() {
// cout << fixed << setprecision(20);
// auto start = std::chrono::steady_clock::now(); // since(start).count()
cin.tie(0)->sync_with_stdio(0);
int n, from, to;
cin >> n;
fo(i, n-1){
cin >> from >> to;
adj[--from].pb(--to);
adj[to].pb(from);
}
fo(i, n) cin >> cameraOn[i];
memset(memo, -1, sizeof(memo));
ll res = min(dp(0, -1, 1), dp(0, -1, 0));
cout << (res>n?"impossible":to_string(res));
return 0;
}
