#include <bits/stdc++.h> using namespace std; using ll = long long; ll const INF = 1e9; int main() { ios_base::sync_with_stdio(false); cin.tie(nullptr); int n; cin >> n; vector<vector<int>> g(n+1); for (int i = 0; i < n-1; i++) { int a, b; cin >> a >> b; g[a].push_back(b); g[b].push_back(a); } vector<bool> on(n+1); for (int i = 1; i <= n; i++) { int on_i; cin >> on_i; on[i] = on_i; } // dp[v][a][b] - zakladajac, ze wierzcholek v jest a i zosatal nacisniety jesli b vector<vector<vector<ll>>> dp(n+1, vector<vector<ll>>(2, vector<ll>(2))); function<void(int, int)> dfs = [&](int u, int p) -> void { int children = 0; for (auto v : g[u]) { if (v == p) continue; dfs(v, u); children++; } if (!children) { dp[u][0][0] = 0; dp[u][0][1] = INF; dp[u][1][0] = INF; dp[u][1][1] = 1; return; } vector<pair<bool, bool>> pos = {{false, false}, {false, true}, {true, false}, {true, true}}; for (auto[a, b] : pos) { ll res = b; int pressed = b; for (auto v : g[u]) { if (v == p) continue; if (dp[v][on[v] != b][1] < dp[v][on[v] != b][0]) { res += dp[v][on[v] != b][1]; pressed++; } else res += dp[v][on[v] != b][0]; } if (res < INF && pressed % 2 != a) { bool all_inf = true; for (auto v : g[u]) { if (v == p) continue; if (dp[v][on[v] != b][0] != INF && dp[v][on[v] != b][1] != INF) all_inf = false; } if (all_inf) res = INF; ll min_dif = INF; for (auto v : g[u]) { if (v == p) continue; min_dif = min(min_dif, abs(dp[v][on[v] != b][1] - dp[v][on[v] != b][0])); } res += min_dif; } dp[u][a][b] = min(res, INF); } }; dfs(1, 0); ll ans = min(dp[1][on[1]][0], dp[1][on[1]][1]); if (ans >= INF) cout << "impossible"; else cout << ans; }