#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pi pair<int, int>
#define pii pair<int, pi>
#define fi first
#define se second
#ifdef _WIN32
#define getchar_unlocked _getchar_nolock
#endif
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
int memo[100005][2][2], n;
bool A[100005];
vector <int> adj[100005];
bool cmp(pi a, pi b){
return a.fi - a.se < b.fi - b.se;
}
int dp(int x, int par, int f, int f2){
if(memo[x][f][f2] != -1)return memo[x][f][f2];
int ok = A[x];
if(f)ok = !ok;
if(f2)ok = !ok;
int ans = 1e18;
vector <pi> cand;
int stuf = 0;
for(auto i : adj[x]){
if(i == par)continue;
int x1 = dp(i, x, 1, f);
int x2 = dp(i, x, 0, f);
cand.push_back({x1, x2});
stuf += x2;
}
sort(cand.begin(), cand.end(), cmp);
for(int i=ok-1;i<(int)cand.size();i+=2){
if(i > 0)stuf += cand[i-1].fi - cand[i-1].se;
if(i >= 0)stuf += cand[i].fi - cand[i].se;
ans = min(ans, stuf);
}
if(f)ans++;
return memo[x][f][f2] = ans;
}
void solve(){
cin >> n;
for(int i=1;i<n;i++){
int a, b; cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for(int i=1;i<=n;i++)cin >> A[i];
memset(memo, -1, sizeof(memo));
int tmp = min(dp(1, -1, 0, 0), dp(1, -1, 1, 0));
if(tmp > n)cout << "impossible";
else cout << tmp;
}
main(){
ios::sync_with_stdio(0);cin.tie(0);
int tc = 1;
//cin >> tc;
for(int tc1=1;tc1<=tc;tc1++){
// cout << "Case #" << tc1 << ": ";
solve();
}
}
