#include <bits/stdc++.h>
using namespace std;
#define int long long
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
#define pb push_back
#define S second
#define F first
#define all(x) (x).begin(),(x).end()
#define migmig ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define deb(x) cerr << (#x) << " " << (x) << endl
//#define lc id << 1
//#define rc lc|1
//#define mid ((l + r)/2)
const int maxn = 1e5 + 10;
//const int maxk = 101;
const int mod = 998244353;
const int inf = 1e9 + 1;
const int LOG = 21;
int n, a[maxn], dp[maxn][2][2];
vector<int> G[maxn];
void dfs(int v, int p) {
if (a[v] == 0) {
dp[v][1][1] = 1;
dp[v][1][0] = inf;
dp[v][0][0] = 0;
dp[v][0][1] = inf;
}
else {
dp[v][1][1] = inf;
dp[v][1][0] = 1;
dp[v][0][0] = inf;
dp[v][0][1] = 0;
}
for (int u : G[v]) {
if (u != p) {
dfs(u, v);
int x = dp[v][0][0], y = dp[v][0][1];
dp[v][0][0] = min(x + dp[u][0][0], y + dp[u][1][0]);
dp[v][0][1] = min(x + dp[u][1][0], y + dp[u][0][0]);
x = dp[v][1][0], y = dp[v][1][1];
dp[v][1][0] = min(x + dp[u][0][1], y + dp[u][1][1]);
dp[v][1][1] = min(x + dp[u][1][1], y + dp[u][0][1]);
}
}
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++) {
dp[v][i][j] = min(inf, dp[v][i][j]);
}
}
}
signed main() {
migmig;
cin >> n;
for (int i = 1; i < n; i++) {
int v, u;
cin >> v >> u;
G[v].pb(u);
G[u].pb(v);
}
for (int i = 1; i <= n; i++)
cin >> a[i];
dfs(1, 0);
int ans = min(dp[1][0][0], dp[1][1][0]);
if (ans >= inf) {
cout << "impossible" << '\n';
}
else {
cout << ans << '\n';
}
}
