#include <bits/stdc++.h>
#define ll long long
#define ld long double
#define sp ' '
#define en '\n'
#define smin(a, b) a = min(a, b)
#define smax(a, b) a = max(a, b)
using namespace std;
const int N = 1e5 + 2;
const int inf = 1e9;
int dp[N][2][2], a[N];
vector<int> g[N];
void Dfs(int s, int e) {
for (int j = 0; j < 4; j++) dp[s][j / 2][j % 2] = inf;
if (g[s].size() == 1 && g[s][0] == e) {
dp[s][a[s]][0] = 0;
dp[s][1 - a[s]][1] = 1;
return;
}
for (int u : g[s]) if (u != e) Dfs(u, s);
int cnt, sum, mn, moze;
cnt = sum = 0;
mn = inf;
moze = 1;
for (int u : g[s]) {
if (u == e) continue;
sum += min(dp[u][0][0], dp[u][0][1]);
if (min(dp[u][0][0], dp[u][0][1]) == inf) moze = 0;
smin(mn, abs(dp[u][0][0] - dp[u][0][1]));
if (dp[u][0][0] > dp[u][0][1]) cnt += 1;
}
if (moze == 1) {
dp[s][0][0] = sum;
if (mn <= 1000000) dp[s][1][0] = sum + mn;
if ((cnt + a[s]) % 2 == 1) swap(dp[s][0][0], dp[s][1][0]);
}
cnt = sum = 0;
mn = inf;
moze = 1;
for (int u : g[s]) {
if (u == e) continue;
sum += min(dp[u][1][0], dp[u][1][1]);
if (min(dp[u][1][0], dp[u][1][1]) == inf) moze = 0;
smin(mn, abs(dp[u][1][0] - dp[u][1][1]));
if (dp[u][1][0] > dp[u][1][1]) cnt += 1;
}
if (moze == 1) {
dp[s][0][1] = sum + 1;
if (mn <= 1000000) dp[s][1][1] = sum + mn + 1;
if ((cnt + a[s]) % 2 == 0) swap(dp[s][0][1], dp[s][1][1]);
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n; cin >> n;
for (int i = 0; i < n - 1; i++) {
int u, v;
cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
for (int i = 1; i <= n; i++) cin >> a[i];
Dfs(1, 0);
int ans = min(dp[1][0][0], dp[1][0][1]);
if (ans == inf) cout << "impossible" << en;
else cout << ans << en;
return 0;
}
