#include<bits/stdc++.h>
#define f first
#define s second
#define int long long
using namespace std;
const int N=2e5+5,mod=1e9+7,Inf=1e8;
int t,dp[N][2][2],n,s[N];
vector<int> V[N];
vector<pair<int,int> > v,v2;
void dfs(int u,int par){
for(int k=0;k<2;k++) {
for(int j=0;j<2;j++) {
dp[u][k][j] = Inf;
}
}
vector<pair<int,int> > v,v2;
int sum = 0,sum2 = 0,p = 0, p2 = 0;
for(int i=0;i<V[u].size();i++){
if(V[u][i] == par) continue;
dfs(V[u][i],u);// cout<<u<<"--->"<<V[u][i]<<endl;
v.push_back( {dp[V[u][i]][1][1] - dp[V[u][i]][1][0],dp[V[u][i]][1][0]});
v2.push_back({dp[V[u][i]][0][1] - dp[V[u][i]][0][0],dp[V[u][i]][0][0]});
sum += dp[V[u][i]][1][0]; sum2+= dp[V[u][i]][0][0];
}
sort(v.begin(),v.end());
sort(v2.begin(),v2.end());
dp[u][1-s[u]][1] = sum2 + 1;
dp[u][s[u]][0] = sum;
//cout<<u<<" "<<sum<<" "<<sum2<<" "<<v.size()<<" "<<dp[u][0][0]<<endl;
for(int j=0;j<v.size();j++) {
p += v[j].f;p2 += v2[j].f;
if(j%2==1)dp[u][s[u]][0] = min(dp[u][s[u]][0],sum+ p);
else dp[u][1^s[u]][0] = min(dp[u][1^s[u]][0],sum + p);
// if(u==4) cout<<sum2<<"__"<<p2<<endl;
if(j%2==1)dp[u][1^s[u]][1] = min(dp[u][1^s[u]][1],sum2 + p2 + 1);
else dp[u][s[u]][1] = min(dp[u][s[u]][1],sum2 + p2 + 1);
}
//cout<<dp[u][1][1]<<endl;
if(par && V[u].size()==1) {
dp[u][s[u]][0] = 0;
dp[u][1-s[u]][1] = 1;
}
//cout<<u<<"___"<<dp[u][1][0] << " "<<dp[u][1][1] << " "<<dp[u][0][0] <<" "<<dp[u][0][1]<<endl;
}
main(){
cin>>n;
for(int i=2;i<=n;i++){
int u,v;
cin>>u>>v;
V[u].push_back(v);
V[v].push_back(u);
}
for(int i=1;i<=n;i++) cin>>s[i],s[i]=1-s[i];
dfs(1,0);
if(min(dp[1][1][1],dp[1][1][0]) >= Inf) cout<<"impossible";
else cout<<min(dp[1][1][1],dp[1][1][0]) ;
}
