#include<iostream>
#include<stdio.h>
#include<math.h>
#include<map>
#include<string>
#include<algorithm>
#include<vector>
#include<string.h>
#include<utility>
#include<set>
#include<cmath>
#include<queue>
#include<deque>
#include<functional>
#include<stack>
#include<limits.h>
#include<iomanip>
#include<unordered_map>
#include<numeric>
#include<tuple>
using namespace std;
#define MOD 1000000007
#define int long long
#define ff first
#define ss second
#define endl '\n'
void solve(){
int n; cin>>n;
vector<int> v[n],c(n);
for(int i=1; i<n; i++){
int a,b; cin>>a>>b,a--,b--;
v[a].push_back(b);
v[b].push_back(a);
}
int ans=1e9;
for(int i=0; i<n; i++) cin>>c[i];
for(int i=0; i<(1<<n); i++){
vector<int> tmp=c;
for(int j=0; j<n; j++){
if(i&(1<<j)){
tmp[j]^=1;
for(auto node: v[j]){
tmp[node]^=1;
}
}
}
int ok=1;
for(int j=0; j<n; j++){
if(tmp[j]) ok=0;
}
int move=__builtin_popcount(i);
if(ok) ans=min(ans,move);
}
if(ans==1e9) cout<<"impossible";
else cout<<ans;
}
signed main(){
// freopen("248.in", "r", stdin);
// freopen("248.out", "w", stdout);
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t=1;
// cin>>t;
while(t--){
solve();
cout<<endl;
}
return 0;
}
