#include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/trie_policy.hpp>
// #include <ext/rope>
using namespace std;
// using namespace __gnu_cxx;
// using namespace __gnu_pbds;
void Hollwo_Pelw();
signed main(){
#ifndef hollwo_pelw_local
if (fopen(".inp", "r"))
assert(freopen(".inp", "r", stdin)), assert(freopen(".out", "w", stdout));
#else
using namespace chrono;
auto start = steady_clock::now();
#endif
cin.tie(0), cout.tie(0) -> sync_with_stdio(0);
int testcases = 1;
// cin >> testcases;
for (int test = 1; test <= testcases; test++){
// cout << "Case #" << test << ": ";
Hollwo_Pelw();
}
#ifdef hollwo_pelw_local
auto end = steady_clock::now();
cout << "\nExecution time : " << duration_cast<milliseconds> (end - start).count() << "[ms]" << endl;
#endif
}
const int N = 1e5 + 5, inf = 1e9;
int n, a[N], dp[N][2][2];
vector<int> adj[N];
void dfs(int u, int p) {
for (int i = 0; i < 2; i++)
dp[u][i][a[u] ^ i] = i;
for (int v : adj[u]) if (v != p) {
dfs(v, u);
for (int i = 0; i < 2; i++) {
int dpu0 = dp[u][i][0], dpu1 = dp[u][i][1];
dp[u][i][0] = min({dpu0 + dp[v][0][i], dpu1 + dp[v][1][i], inf});
dp[u][i][1] = min({dpu0 + dp[v][1][i], dpu1 + dp[v][0][i], inf});
}
}
}
void Hollwo_Pelw(){
memset(dp, 0x3f, sizeof dp);
cin >> n;
for (int i = 1, u, v; i < n; i++) {
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int i = 1; i <= n; i++)
cin >> a[i];
dfs(1, 0);
int res = min(dp[1][0][0], dp[1][1][0]);
if (res > n)
cout << "impossible\n";
else
cout << res << '\n';
}
