#include <iostream>
#include <vector>
using namespace std;
const int N = 1<<18;
vector<int> nei[N];
int dp[N][4], stt[N];
void dfs(int u, int p){
dp[u][1] = dp[u][3] = N;
for (int i : nei[u]){
if (i == p)
continue;
dfs(i, u);
int a[] = {dp[u][0], dp[u][1], dp[u][2], dp[u][3]};
dp[u][0] = min(a[0] + dp[i][0], a[1] + dp[i][1]);
dp[u][1] = min(a[1] + dp[i][0], a[0] + dp[i][1]);
dp[u][2] = min(a[2] + dp[i][2], a[3] + dp[i][3]);
dp[u][3] = min(a[3] + dp[i][2], a[2] + dp[i][3]);
}
int a[] = {dp[u][0], dp[u][1], dp[u][2], dp[u][3]};
// cout<<u<<" ::\n";
// cout<<a[0]<<" "<<a[1]<<" "<<a[2]<<" "<<a[3]<<endl;
if (stt[u]){
dp[u][0] = a[0|1];
dp[u][1] = a[2|0] + 1;
dp[u][2] = a[0|0];
dp[u][3] = a[2|1] + 1;
}
else{
dp[u][0] = a[0|0];
dp[u][1] = a[2|1] + 1;
dp[u][2] = a[0|1];
dp[u][3] = a[2|0] + 1;
}
// cout<<dp[u][0]<<" "<<dp[u][1]<<" "<<dp[u][2]<<" "<<dp[u][3]<<endl;
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(NULL);
int n;
cin>>n;
for (int i=1, a, b;i<n;i++){
cin>>a>>b;
nei[a].push_back(b);
nei[b].push_back(a);
}
for (int i=1;i<=n;i++)
cin>>stt[i];
dfs(1, 1);
int Ans = min(dp[1][0], dp[1][1]);
if (Ans >= N)
cout<<"impossible\n";
else
cout<<Ans<<'\n';
}
