#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1000000;
int N, L[MAXN], dp[MAXN][2][2];
vector<int> e[MAXN];
void dfs(int i = 0, int p = -1) {
for (int s = 0; s < 2; s++) {
dp[i][s][L[i] ^ s] = s;
dp[i][s][L[i] ^ s ^ 1] = N + 1;
}
for (int j : e[i])
if (j != p) {
dfs(j, i);
for (int s = 0; s < 2; s++) {
int dp0 = dp[i][s][0], dp1 = dp[i][s][1];
dp[i][s][0] = min(N + 1, min(dp0 + dp[j][0][s], dp1 + dp[j][1][s]));
dp[i][s][1] = min(N + 1, min(dp0 + dp[j][1][s], dp1 + dp[j][0][s]));
}
}
}
int main() {
cin >> N;
int A, B;
for (int i = 0; i < N - 1; i++) {
cin >> A >> B;
A--, B--;
e[A].push_back(B);
e[B].push_back(A);
}
for (int i = 0; i < N; i++)
cin >> L[i];
dfs();
int r = min(dp[0][0][0], dp[0][1][0]);
if (r > N)
cout << "impossible\n";
else
cout << r << "\n";
}
