/* In the name of God, aka Allah */
// let this be mytemp.cpp
#include<bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define endl '\n'
#define mk make_pair
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int N = 1e5 + 50;
int n, c[N], dp[N][2][2];
vector<int> adj[N];
void dfs(int u, int p) {
dp[u][0][0] = dp[u][0][1] = dp[u][1][0] = dp[u][1][1] = N;
int s0 = 0, s1 = 1, m0 = N, m1 = N, c0 = c[u], c1 = c[u]^1;
for (int v:adj[u]) {
if (v == p) continue;
dfs(v, u);
s0 += min(dp[v][0][0], dp[v][0][1]);
s1 += min(dp[v][1][0], dp[v][1][1]);
c0 ^= (dp[v][0][1] < dp[v][0][0]);
c1 ^= (dp[v][1][1] < dp[v][1][0]);
m0 = min(m0, abs(dp[v][0][0]-dp[v][0][1]));
m1 = min(m1, abs(dp[v][1][0]-dp[v][1][1]));
}
dp[u][c0][0] = min(dp[u][c0][0], s0);
dp[u][c0^1][0] = min(dp[u][c0^1][0], s0+m0);
dp[u][c1][1] = min(dp[u][c1][1], s1);
dp[u][c1^1][1] = min(dp[u][c1^1][1], s1+m1);
}
void solve() {
cin >> n;
for (int i = 1; i <= n-1; i++) {
int u, v;
cin >> u >> v;
adj[u].push_back(v), adj[v].push_back(u);
}
for (int u = 1; u <= n; u++) {
cin >> c[u];
}
dfs(1, 0);
int ans = min(dp[1][0][0], dp[1][0][1]);
if (ans >= N) return cout << "impossible" << endl, void();
cout << ans << endl;
}
int main() {
ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
solve();
return 0;
}
