#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
int n;
cin >> n;
vector<vector<int>> adj(n);
for (int i = 0, u,v ; i < n - 1; i++){
cin >> u >> v;
u--, v--;
adj[u].push_back(v);
adj[v].push_back(u);
}
vector<int> t(n);
for (int i = 0; i < n; i++){
cin >> t[i];
}
vector<vector<vector<i64>>> dp(2, vector<vector<i64>>(2, vector<i64>(n, -1)));
auto dfs = [&](auto &&me, int u, int p) -> void{
for (int v : adj[u]){
if (v == p){
continue;
}
me(me, v, u);
}
for (int tg = 0; tg < 2; tg++){
for (int fl = 0; fl < 2; fl++){
i64 sure0 = 0, sure1 = 0;
int uneq = t[u] ^ tg ^ fl;
int count1 = 0;
vector<pair<i64,i64>> unsure;
bool ok = true;
for (int v : adj[u]){
if (v == p){
continue;
}
bool k0 = dp[fl][0][v] != -1, k1 = dp[fl][1][v] != -1;
if (!k0 && !k1){
ok = false;
break;
}
else if (!k0){
sure1 += dp[fl][1][v];
count1++;
}
else if (!k1){
sure0 += dp[fl][0][v];
}
else{
unsure.push_back({dp[fl][0][v], dp[fl][1][v]});
}
}
if (!ok){
dp[tg][fl][u] = -1;
continue;
}
if (unsure.empty()){
if (count1 % 2 != uneq){
dp[tg][fl][u] = -1;
}
else{
dp[tg][fl][u] = sure0 + sure1 + fl;
}
}
else{
i64 sum = 0;
vector<i64> ch;
for (auto &[a, b] : unsure){
sum += a;
ch.push_back(b - a);
}
sort(ch.begin(), ch.end());
i64 ans = LLONG_MAX;
int take = 0;
while (take < uneq){
sum += ch[take];
take++;
}
ans = min(ans, sum);
for (int i = uneq; i < unsure.size(); i += 2){
while (take < i){
sum += ch[take];
take++;
}
ans = min(ans, sum);
}
dp[tg][fl][u] = ans + fl;
}
}
}
};
dfs(dfs, 0, 0);
if (dp[0][0][0] == -1 && dp[0][1][0] == -1){
cout << "impossible\n";
}
else {
if (dp[0][0][0] == -1){
dp[0][0][0] = LLONG_MAX;
}
if (dp[0][1][0] == -1){
dp[0][1][0] = LLONG_MAX;
}
cout << min(dp[0][0][0], dp[0][1][0]) << '\n';
}
}
