#include <bits/stdc++.h>
using namespace std;
#define int long long
const int maxn = 1E5+5;
const int INF = 1E9;
int n, dp[maxn][2][2], b[maxn]; // colours AFTER flipping, times flipped
bool cn[maxn][2][2];
vector<int> adj[maxn];
void dfs(int u, int p) {
// cout << u << " " << p << endl;
for (int i = 0; i < 2; i++) for (int j = 0; j < 2; j++) {
dp[u][i][j] = INF;
}
// int one[2][2];
// for (int i = 0; i < 2; i++) {
// for (int j = 0; j < 2; j++) {
// cn[u][i][j] = (b[u] == (i ^ j));
// one[i][j] = 0;
// }
// }
int child = 0;
for (int v : adj[u]) {
if (v == p) continue;
dfs(v, u);
child++;
// cn[u][0][0] = (cn[u][0][0] & cn[v][0][0]);
// cn[u][1][0] = (cn[u][1][0] & cn[v][0][0]);
// cn[u][0][1] = (cn[u][0][1] & cn[v][1][0]);
// cn[u][1][1] = (cn[u][1][1] & cn[v][1][0]);
}
// for (int i = 0; i < 2; i++) {
// for (int j = 0; j < 2; j++) {
// if (!cn[u][i][j]) continue;
// for (int v : adj[u]) {
// if (v == p) continue;
// dp[u][i][j] += dp[v][j][0] + j;
// }
// }
// }
for (int k = 0; k < 2; k++) {
int i = 0, one[2][2];
one[0][0] = one[1][0] = 0;
one[0][1] = one[1][1] = INF;
for (int v : adj[u]) {
if (v == p) continue;
one[i][0] = min({one[i ^ 1][1] + dp[v][k][1], one[i ^ 1][0] + dp[v][k][0]});
one[i][1] = min({one[i ^ 1][0] + dp[v][k][1], one[i ^ 1][1] + dp[v][k][0]});
// cout << "ones: " << one[i][0] << " " << one[i][1] << "\n";
i = i ^ 1;
}
// cout << (i ^ 1) << " " << k << " " << one[0][0] << " " << one[0][1] << " " << one[1][0] << " " << one[1][1] << "\n";
// cout << (i ^ 1) << " " << (b[u] ^ k) << endl;
dp[u][b[u] ^ k][k] = min(dp[u][b[u] ^ k][k], one[i ^ 1][0]);
dp[u][b[u] ^ 1 ^ k][k] = min(dp[u][b[u] ^ 1 ^ k][k], one[i ^ 1][1]);
}
dp[u][0][1]++, dp[u][1][1]++;
// for (int i = 0; i < 2; i++) {
// for (int j = 0; j < 2; j++) {
// if (b[u] != (i ^ j)) dp[u][i][j] = INF;
// }
// }
// cout << u << endl;
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++) {
// cout << ((i == 0) ? "off " : "on ") <<((j == 0) ? "noflip " : "flip ") << dp[u][i][j] << ' ';
}
// cout << '\n';
}
}
int32_t main() {
ios::sync_with_stdio(0); cin.tie(0);
cin >> n;
for (int i = 1; i < n; i++) {
int u, v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int i = 1; i <= n; i++) cin >> b[i];
dfs(1, 0);
int mn = 1E18;
for (int j = 0; j < 2; j++) {
mn = min(mn, dp[1][0][j]);
}
if (mn > n) cout << "impossible\n";
else cout << mn << endl;
}
