#include<bits/stdc++.h>
using namespace std;
#define mp make_pair
#define INF 10000000
#define MOD ((ll)1000000007)
#define MID ((l+r)/2)
#define HASHMOD 2305843009213693951
#define ll long long
#define ull unsigned long long
#define F first
#define S second
typedef pair<ll, ll> ii;
typedef pair<ii, int> iii;
typedef vector<int> vi;
typedef vector<ii> vii;
typedef map<int, int> mii;
#define EPS 1e-6
#define FOR(i,n) for(int i=0;i<((int)(n));i++)
#define FORi(i,a,b) for(int i=((int)(a));i<((int)(b));i++)
#define FOA(v, a) for(auto v : a)
int t, n;
vi a, b;
vector<vi> adj;
ll dp[200000][2][2];
ll dfs(int s, int x, int xp, int p=0){
if(dp[s][x][xp]!=-1) return dp[s][x][xp];
ll ans=0;
int c = a[s];
if(xp) c = 1-c;
if(x) c = 1-c, ans++;
int ccnt=0;
FOA(v, adj[s]) if(v!=p) ccnt++;
if(ccnt==1){
//cout<<s<<" "<<x<<" "<<xp<<" C 1\n";
FOA(v, adj[s]){
if(v!=p) ans+= dfs(v, c, x, s);
}
}
else if(ccnt==2){
ll a1=0, a2=0;
int cnt=0;
FOA(v, adj[s]){
if(v==p) continue;
a1+= dfs(v, cnt, x, s);
a2+= dfs(v, 1-cnt, x, s);
if(c) cnt = 1-cnt;
}
ans += min(a1, a2);
}
else if(ccnt==0 && c!=0) ans = MOD;
else if(ccnt>2) return MOD;
dp[s][x][xp] = ans;
//cout<<s<<" "<<x<<" "<<xp<<": "<<ans<<endl;
return ans;
}
int main(){
cin>>n;
adj.assign(n+1, vi());
FOR(i,n+2) dp[i][0][0] = dp[i][0][1] = dp[i][1][0] = dp[i][1][1] = -1;
FOR(i,n-1){
int a, b;
cin>>a>>b;
adj[a].push_back(b);
adj[b].push_back(a);
}
a.resize(n+1);
a[0] = 0;
FOR(i,n){
cin>>a[i+1];
}
ll ans1 = dfs(1, 0, 0);
ll ans2 = dfs(1, 1, 0);
if(ans1 >= MOD && ans2 >= MOD) cout<<"impossible\n";
else cout<<min(ans1, ans2)<<endl;
}
