#include <bits/stdc++.h>
using namespace std;
#define int long long
vector<int> adj[100005];
int dp[100005][2];
int state[100005];
void solve(int n, int p) {
for (auto i:adj[n]) {
if (i!=p) solve(i,n);
}
for (int i = 0; i < 2; i++) {
int newstate = state[n]^i;
//by default, take all of that type first
//you want to take newstate%2 such shit.
vector<int> diff;
int total = 0; for (auto x:adj[n]) if (x!=p) total += dp[x][0],diff.push_back(dp[x][1]-dp[x][0]);
sort(diff.begin(), diff.end());
int start=0;
int ans=2e9;
if (newstate==1 and diff.size()==0) {dp[n][i]=2e9;continue;}
else if (diff.size()==0) {dp[n][i]=i; continue;}
else if (newstate==1){
total += diff[0];
ans=total;
start=1;
}
for (; start < diff.size()-1; start+=2) {
total += diff[start];
total += diff[start+1];
ans=min(ans,total);
}
dp[n][i]=min(2000000000ll,ans+i);
}
}
signed main() {
int n; cin >> n;
for (int i = 0; i < n-1; i++) {
int a,b; cin >> a>> b;
a--;b--;
adj[a].push_back(b);
adj[b].push_back(a);
}
for (int i = 0; i < n; i++) cin >> state[i];
solve(0,-1);
int ans=min(dp[0][0], dp[0][1]);
if (ans==2e9) cout << "impossible";
else cout << ans;
}
