#include <bits/stdc++.h>
using namespace std;
#define int long long
typedef pair<int,int> pii;
vector<vector<int>> adj;
vector<bool> state;
signed main() {
int n; cin>>n;
adj.resize(n);
state.resize(n);
for(int i=0;i<n-1;i++) {
int u,v; cin>>u>>v; u--; v--;
adj[u].push_back(v);
adj[v].push_back(u);
}
for(int i=0;i<n;i++) {
int x; cin>>x;
if(x==1)state[i]=true;
else state[i]=false;
}
int ans=INT_MAX;
for(int i=0;i<(1<<n);i++) {
vector<bool> s = state;
int cnt=0;
for(int j=0;j<n;j++) {
if(i&(1<<j)) {
s[j]=!s[j];
for(int x: adj[j]) s[x]=!s[x];
cnt++;
}
}
bool all_off=true;
for(bool x:s){
if(x)all_off=false;
}
if(all_off) ans=min(ans,cnt);
}
if(ans==INT_MAX)cout<<"impossible";
else cout<<ans;
return 0;
}
