#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
vector<vector<int>> adj(n);
vector<int> state(n);
for (int i = 1; i < n; ++i) {
int a, b;
cin >> a >> b;
--a; --b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for (auto& x : state) cin >> x;
vector<vector<vector<int>>> dp(n,vector<vector<int>>(2, vector<int>(2, -1)));
auto dfs = [&](auto& dfs, int i, int p, int on, int toggle) -> int {
if (dp[i][on][toggle] != -1) return dp[i][on][toggle];
array<int, 2> now = {n + 1, n + 1};
now[state[i]] = 0;
for (auto& j : adj[i]) {
if (j == p) continue;
array<int, 2> ndp;
// dont turn on
ndp[0] = dfs(dfs, j, i, toggle, 0);
// turn on
ndp[1] = dfs(dfs, j, i, toggle, 1);
auto nnow = now;
nnow[0] = min({n + 1, now[0] + ndp[0], now[1] + ndp[1]});
nnow[1] = min({n + 1, now[0] + ndp[1], now[1] + ndp[0]});
now = nnow;
}
return dp[i][on][toggle] = now[(on ^ toggle)] + toggle;
};
int res = min(dfs(dfs, 0, -1, 0, 0), dfs(dfs, 0, -1, 0, 1));
if (res > n) {
cout << "impossible\n";
return 0;
}
cout << res << endl;
}
