#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using pii = pair<int, int>;
#define pb push_back
#define ff first
#define ss second
const int inf = 1e9;
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n; cin>>n;
vector<int> g[n + 1];
for (int i = 1; i < n; i++){
int x, y; cin>>x>>y;
g[x].pb(y);
g[y].pb(x);
}
vector<int> a(n + 1);
for (int i = 1; i <= n; i++){
cin>>a[i];
}
int dp[n + 1][2][2];
function<void(int, int)> dfs = [&](int v, int pr){
for (int i: g[v]){
if (i == pr) continue;
dfs(i, v);
}
vector<int> all;
ll tot = 0;
for (int i: g[v]){
if (i == pr) continue;
tot += dp[i][a[i]][0];
all.pb(dp[i][a[i]][1] - dp[i][a[i]][0]);
}
sort(all.begin(), all.end());
vector<ll> p(all.size() + 1);
for (int i = 1; i <= all.size(); i++){
p[i] = p[i - 1] + all[i - 1];
}
vector<int> mn(2, inf);
for (int i = 0; i <= all.size(); i++){
mn[i % 2] = (int) min((ll) mn[i % 2], tot + p[i]);
}
dp[v][0][0] = mn[0];
dp[v][1][0] = mn[1];
all.clear();
tot = 0;
for (int i: g[v]){
if (i == pr) continue;
tot += dp[i][!a[i]][0];
all.pb(dp[i][!a[i]][1] - dp[i][!a[i]][0]);
}
sort(all.begin(), all.end());
for (int i = 1; i <= all.size(); i++){
p[i] = p[i - 1] + all[i - 1];
}
mn[0] = mn[1] = inf;
for (int i = 0; i <= all.size(); i++){
mn[i % 2] = (int) min((ll) mn[i % 2], tot + p[i] + 1);
}
dp[v][0][1] = mn[1];
dp[v][1][1] = mn[0];
};
dfs(1, 0);
int out = min(dp[1][a[1]][0], dp[1][a[1]][1]);
if (out == inf){
cout<<"impossible"<<"\n";
}
else {
cout<<out<<"\n";
}
}
