#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
ll n,cur[100005];
vector <ll> al[100005];
ll vis[100005];
ll dp[100005][2][2];
void dfs(ll node){
vis[node]=1;
if (cur[node]==1){
dp[node][1][0]+=1;
dp[node][0][1]+=0;
dp[node][0][0]+=1e9;
dp[node][1][1]+=1e9;
}
else{
dp[node][1][0]+=1e9;
dp[node][0][1]+=1e9;
dp[node][0][0]+=0;
dp[node][1][1]+=1;
}
for (auto u:al[node]){
if (vis[u]!=1){
vis[u]=1;
dfs(u);
ll x=dp[node][0][0], y=dp[node][0][1];
dp[node][0][0]=min(x+dp[u][0][0],y+dp[u][1][0]);
dp[node][0][1]=min(y+dp[u][0][0],x+dp[u][1][0]);
ll a=dp[node][1][0],b=dp[node][1][1];
dp[node][1][0]=min(a+dp[u][0][1],b+dp[u][1][1]);
dp[node][1][1]=min(b+dp[u][0][1],a+dp[u][1][1]);
}
}
}
int main(){
cin>>n;
for (int i=1;i<n;i++){
ll a,b;
cin>>a>>b;
al[a].push_back(b);
al[b].push_back(a);
}
for (int i=1;i<=n;i++){
cin>>cur[i];
}
dfs(1);
ll ans=min(dp[1][0][0],dp[1][1][0]);
if (ans>n) cout<<"impossible";
else cout<<ans;
}
