#include<bits/stdc++.h>
#define int long long
using namespace std;
vector<vector<int> > n1;
vector<int> n2;
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin>>n;
n1.resize(n);
n2.resize(n);
for (int i = 0;i<n-1;i++){
int a,b;
cin>>a>>b;
n1[a-1].push_back(b-1);
n1[b-1].push_back(a-1);
}
for (int i = 0;i<n;i++) cin>>n2[i];
int ans = 1e9;
for (int i = 0;i<(1<<n);i++){
int a = 0;
bool hi = true;
for (int j = 0;j<n;j++){
int x = 0;
for (int k = 0;k<n1[j].size();k++){
if (i>>(n1[j][k])&1) x++;
}
if (i>>j &1) x++;
if (x%2!=n2[j]){ hi = false;break;}
}
if (hi){
for (int j = 0;j<n;j++){
if (i>>j & 1) a++;
}
ans = min(ans,a);
}
}
if (ans==1e9) cout<<"impossible";
else cout<<ans;
return 0;
}
