#include <bits/stdc++.h>
using namespace std;
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
vector v(n + 1, vector<int>());
for (int i = 1; i < n; i++) {
int r1, r2;
cin >> r1 >> r2;
v[r1].emplace_back(r2);
v[r2].emplace_back(r1);
}
vector<int> a(n + 1);
for (int i = 1; i <= n; i++) {
cin >> a[i];
}
vector dp(n + 5, vector(4, (long long)1e18));
// dp[0] = no choose self, 0
// dp[1] = choose self, 0
// dp[2] = no choose self, 1
// dp[3] = choose self, 1
// choose x:
// each child choose min(dp[2], dp[3])
// each child try swap
// update dp[1], dp[3]
// no choose x:
// each child choose min(dp[0], dp[1])
// each child try swap
// update dp[0], dp[2]
//problem of only one child?
auto dfs = [&](auto&& dfs, int x, int last) -> void {
if (v[x].size() == 1 && last != -1) {
if (a[x] == 0) {
dp[x][0] = 0;
dp[x][1] = 1e9;
dp[x][2] = 1e9;
dp[x][3] = 1;
}
else {
dp[x][0] = 1e9;
dp[x][1] = 1;
dp[x][2] = 0;
dp[x][3] = 1e9;
}
return;
}
for (auto i : v[x]) {
if (i == last) continue;
dfs(dfs, i, x);
}
int ct = 1 - a[x];
long long sum = 1;
for (auto i : v[x]) {
if (i == last) continue;
if (dp[i][2] < dp[i][3]) {
sum += dp[i][2];
}
else {
sum += dp[i][3];
ct++;
}
}
long long mn = 1e9;
for (auto i : v[x]) {
if (i == last) continue;
if (dp[i][2] < dp[i][3]) mn = min(mn, sum - dp[i][2] + dp[i][3]);
else mn = min(mn, sum - dp[i][3] + dp[i][2]);
}
if (ct % 2 == 0) {
dp[x][1] = sum;
dp[x][3] = mn;
}
else {
dp[x][3] = sum;
dp[x][1] = mn;
}
ct = a[x];
sum = 0;
for (auto i : v[x]) {
if (i == last) continue;
if (dp[i][0] < dp[i][1]) {
sum += dp[i][0];
}
else {
sum += dp[i][1];
ct++;
}
}
mn = 1e9;
for (auto i : v[x]) {
if (i == last) continue;
if (dp[i][0] < dp[i][1]) mn = min(mn, sum - dp[i][0] + dp[i][1]);
else mn = min(mn, sum - dp[i][1] + dp[i][0]);
}
if (ct % 2 == 0) {
dp[x][0] = sum;
dp[x][2] = mn;
}
else {
dp[x][2] = sum;
dp[x][0] = mn;
}
};
dfs(dfs, 1, -1);
// for (int i = 1; i <= n; i++) {
// cout << i << ":\n";
// for (int j = 0; j <= 3; j++) {
// cout << " " << j << ": " << dp[i][j] << endl;
// }
// }
long long ans = min(dp[1][0], dp[1][1]);
if (ans >= 1e9) cout << "impossible\n";
else cout << ans << "\n";
return 0;
}
