#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
typedef vector<vi> vii;
typedef pair<int,int> pi;
typedef vector<pi> pii;
typedef tuple<int,int,int> tii;
typedef vector<ll> li;
typedef vector<li> lii;
#define REP(i,a,b) for(int i=a;i<b;i++)
#define F first
#define S second
#define PB push_back
#define MP make_pair
#define LSOne(s) ((s)&(-s))
ll INF=1e18+10;
int inf=1e9+10;
ll M=1e9+7;
int main() {
//ios::sync_with_stdio(0);
//cin.tie(0);
//freopen("test_input.txt", "r", stdin);
//freopen("test_output.txt", "w", stdout);
int n;cin>>n;
vii a(n);
REP(i,0,n-1){
int x,y;
cin>>x>>y;
x--;y--;
a[x].PB(y);
a[y].PB(x);
}
vi b(n);
REP(i,0,n)cin>>b[i];
int ans=inf;
int k=pow(2,n);
REP(i,0,k){
vi c=b;int d=0;
REP(j,0,n)if(i&(1<<j)){
c[j]=1-c[j];
for(auto u:a[j])c[u]=1-c[u];
d++;
}
bool flag=1;
REP(j,0,n)if(c[j]==1)flag=0;
if(flag)ans=min(ans,d);
}
if(ans==inf)cout<<"impossible";
else cout<<ans;
}
