#include <bits/stdc++.h>
using namespace std;
#define INF 0x3f3f3f3f
#define rep(i, m, n) for (int i = m; i <= n; i++)
#define per(i, m, n) for (int i = m; i >= n; i--)
#define sz(v) (int) v.size()
#define all(v) v.begin(), v.end()
#define ll long long
#define pb push_back
#define mp make_pair
#define fi first
#define se second
const int maxn = 1e5 + 5;
int N;
ll dp[maxn][2][2], a[maxn];
vector<int> G[maxn];
void dfs(int u, int k) {
ll sum1 = 0, sum2 = 0, mn1 = 1e18, mn2 = 1e18, cnt1 = 0, cnt2 = 0;
for (auto v : G[u]) {
if (v == k) continue;
dfs(v, u);
sum1 += min(dp[v][0][0], dp[v][0][1]);
mn1 = min(mn1, abs(dp[v][0][0] - dp[v][0][1]));
if (dp[v][0][1] < dp[v][0][0]) cnt1++;
sum2 += min(dp[v][1][0], dp[v][1][1]);
mn2 = min(mn2, abs(dp[v][1][0] - dp[v][1][1]));
if (dp[v][1][1] < dp[v][1][0]) cnt2++;
}
if ((cnt1 + a[u]) % 2 == 0) {
dp[u][0][0] = min(dp[u][0][0], sum1);
dp[u][1][0] = min(dp[u][1][0], sum1 + mn1);
} else {
dp[u][0][0] = min(dp[u][0][0], sum1 + mn1);
dp[u][1][0] = min(dp[u][1][0], sum1);
}
if ((cnt2 + a[u]) % 2 == 1) {
dp[u][0][1] = min(dp[u][0][1], sum2 + 1);
dp[u][1][1] = min(dp[u][1][1], sum2 + mn2 + 1);
} else {
dp[u][0][1] = min(dp[u][0][1], sum2 + mn2 + 1);
dp[u][1][1] = min(dp[u][1][1], sum2 + 1);
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
cin >> N;
rep(i, 1, N - 1) {
int u, v;
cin >> u >> v;
G[u].pb(v), G[v].pb(u);
}
rep(i, 1, N) cin >> a[i];
rep(i, 1, N) rep(j, 0, 1) rep(k, 0, 1) dp[i][j][k] = 1e9;
dfs(1, 0);
ll ans = min(dp[1][0][0], dp[1][0][1]);
if (ans == (ll) 1e9) {
cout << "impossible\n";
} else cout << ans << "\n";
return 0;
}
