#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
using namespace __gnu_pbds;
typedef tree<long long, null_type, less_equal<>,
rb_tree_tag, tree_order_statistics_node_update>
ordered_set;
#define ll long long
#define ii pair<ll,ll>
#define iii tuple<ll,ll,ll>
#ifndef DEBUG
#define cerr if (0) cerr
#define endl '\n'
#endif
const ll inf=1e15;
const ll maxn=5e5+5;
const ll mod=1e9+7;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
vector<ll> adj[maxn];
ll arr[maxn];
ll dp[maxn][2][2];
ll dfs(ll x, ll p, ll fs, ll fp){
if (dp[x][fs][fp]!=-1) return dp[x][fs][fp];
ll parity=arr[x]^fs^fp;
dp[x][fs][fp]=fs;
priority_queue<ll,vector<ll>,greater<ll>> pq;
for (auto u:adj[x]){
if (u==p) continue;
dfs(u,x,0,fs);
dfs(u,x,1,fs);
dp[x][fs][fp]+=dp[u][0][fs];
pq.emplace(dp[u][1][fs]-dp[u][0][fs]);
}
if (parity==1 && pq.empty()){
return dp[x][fs][fp]=inf;
}
if (parity){
dp[x][fs][fp]+=pq.top();
pq.pop();
}
while (pq.size()){
ll a=pq.top();
pq.pop();
if (pq.empty()) break;
ll b=pq.top();
pq.pop();
if (a+b>=0) break;
dp[x][fs][fp]+=(a+b); //add if we can get negative result
//i.e. savings
}
return dp[x][fs][fp];
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
memset(dp,-1,sizeof(dp));
ll n,a,b;
cin>>n;
for (int i=1;i<n;i++){
cin>>a>>b;
adj[a].emplace_back(b);
adj[b].emplace_back(a);
}
for (int i=1;i<=n;i++) cin>>arr[i];
dfs(1,0,0,0);
dfs(1,0,1,0);
ll ans=min(dp[1][0][0],dp[1][1][0]);
if (ans>=inf){
cout<<"impossible";
} else cout<<ans;
return 0;
}
