#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N = 1e5+5, inf = INT_MAX;
int n, dp[2][2][N]; // me he pulsado a mi mismo?, soy 0 o 1?, nodo
vector<int> adj[N];
bitset<N> bt;
void dfs(int x, int p) {
vector<vector<int>> sum(2, vector<int>(2, 0));
vector<int> mnSum(2, 0);
vector<int> cnt(2, 0);
vector<int> mnDif(2, inf);
for (int& y : adj[x]) {
if (y == p) continue;
dfs(y, x);
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++) {
sum[i][j] += dp[i][j][y];
}
}
for (int i = 0; i < 2; i++) {
if (dp[0][i][y] < dp[1][i][y]) {
mnSum[i] += dp[0][i][y];
mnDif[i] = min(mnDif[i], dp[1][i][y] - dp[0][i][y]);
}
else {
mnSum[i] += dp[1][i][y];
cnt[i]++;
mnDif[i] = min(mnDif[i], dp[0][i][y] - dp[1][i][y]);
}
}
}
if (!bt[x]) {
dp[0][0][x] = mnSum[0] + (cnt[0] % 2 ? mnDif[0] : 0);
dp[0][1][x] = mnSum[0] + (!(cnt[0] % 2) ? mnDif[0] : 0);
dp[1][0][x] = 1 + mnSum[1] + (!(cnt[1] % 2) ? mnDif[1] : 0);
dp[1][1][x] = 1 + mnSum[1] + (cnt[1] % 2 ? mnDif[1] : 0);
}
else {
dp[0][0][x] = mnSum[0] + (!(cnt[0] % 2) ? mnDif[0] : 0);
dp[0][1][x] = mnSum[0] + (cnt[0] % 2 ? mnDif[0] : 0);
dp[1][0][x] = 1 + mnSum[1] + (cnt[1] % 2 ? mnDif[1] : 0);
dp[1][1][x] = 1 + mnSum[1] + (!(cnt[1] % 2) ? mnDif[1] : 0);
}
}
signed main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
for (int i = 0; i < n-1; i++) {
int x, y;
cin >> x >> y;
x--; y--;
adj[x].push_back(y);
adj[y].push_back(x);
}
for (int i = 0; i < n; i++) {
int x;
cin >> x;
if (x) bt[i] = 1;
}
dfs(0, 0);
int ans = min(dp[0][0][0], dp[1][0][0]);
if (ans>= inf) cout << "impossible\n";
else cout << ans << "\n";
}
