// In the name of God
#include<bits/stdc++.h>
using namespace std;
#define pb push_back
#define fi first
#define se second
#define mp make_pair
typedef long long ll;
const int N = 2e5 + 5;
int n;
vector<int> adj[N];
ll dp[N][2][2];
bool a[N];
void dfs(int v, int p) {
ll ans[2] = {0, 0}, cnt[2] = {0, 0};
ll del[2] = {n * 10LL, n * 10LL};
for (auto u : adj[v]) {
if (u == p)
continue;
dfs(u, v);
for (int i = 0; i < 2; i++) {
ans[i] += min(dp[u][0][i], dp[u][1][i]);
del[i] = min(del[i], (ll)abs(dp[u][0][i] - dp[u][1][i]));
cnt[i] += dp[u][1][i] <= dp[u][0][i];
}
}
if (adj[v].size() == 1) {
for (int i = 0; i < 2; i++) {
dp[v][i][a[v] ^ i] = i;
}
return;
}
dp[v][0][a[v]] = ans[0] + ((cnt[0] % 2) ? del[0] : 0LL);
dp[v][0][a[v] ^ 1] = ans[0] + ((cnt[0] % 2) == 0 ? del[0] : 0LL);
dp[v][1][a[v]] = ans[1] + ((cnt[1] % 2) == 0 ? del[1] : 0LL) + 1;
dp[v][1][a[v] ^ 1] = ans[1] + ((cnt[1] % 2) ? del[1] : 0LL) + 1;
}
void solve() {
cin >> n;
for (int i = 0; i< n - 1; i++) {
int v, u; cin >> v >> u;
adj[v].pb(u), adj[u].pb(v);
}
int rt = 1;
for (int i = 1; i <= n; i++) {
cin >> a[i];
if (adj[i].size() > 1)
rt = i;
}
for (int i = 0; i <= n; i++)
for (int j = 0; j < 2; j++)
for (int k = 0; k < 2; k++)
dp[i][j][k] = n * 10;
dfs(rt, 0);
ll ans = min(dp[rt][0][0], dp[rt][1][0]);
if (ans <= n)
cout << ans << endl;
else
cout << "impossible" << endl;
}
int32_t main() {
ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
solve();
return 0;
}
