#include <bits/stdc++.h>
using namespace std;
const int N = 100005, oo = 1e9;
void mini(int &X, int Y) {
if (X >= Y) X = Y;
}
int n;
bool b[N];
vector<int> adj[N];
int dp[N][2][2];
void DFS(int u, int par) {
array<array<array<int, 2>, 2>, 2> tmp;
tmp[0][0][0] = tmp[0][0][1] = 0;
tmp[0][1][0] = tmp[0][1][1] = oo;
int i = 0;
for (int v: adj[u]) if (v != par) {
DFS(v, u);
i++;
int id = i & 1;
for (int k = 0; k < 2; k++) {
tmp[id][k][0] = tmp[id][k][1] = oo;
mini(tmp[id][k][0], min(tmp[id ^ 1][k ^ 1][0] + dp[v][1][0], tmp[id ^ 1][k][0] + dp[v][0][0]));
mini(tmp[id][k][1], min(tmp[id ^ 1][k ^ 1][1] + dp[v][1][1], tmp[id ^ 1][k][1] + dp[v][0][1]));
}
}
dp[u][0][b[u]] = tmp[i & 1][0][0];
dp[u][1][b[u]] = 1 + tmp[i & 1][1][1];
dp[u][0][1 ^ b[u]] = tmp[i & 1][1][0];
dp[u][1][1 ^ b[u]] = 1 + tmp[i & 1][0][1];
}
signed main(void) {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
for (int i = 1, u, v; i < n; i++) {
cin >> u >> v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
for (int i = 1; i <= n; i++) cin >> b[i];
DFS(1, 1);
int ans = min(dp[1][0][0], dp[1][1][0]);
if (ans >= oo) cout << "impossible";
else cout << ans;
return 0;
}
