#include<bits/stdc++.h>
#define int long long
using namespace std;
vector<vector<int> > n1;
vector<pair<pair<int,int>, pair<int,int> > > dp;
vector<int> n2;
void dfs(int x, int p){
if (n2[x]==0) dp[x] = make_pair(make_pair(1,1e9),make_pair(1e9,0));
else dp[x] = make_pair(make_pair(1e9,1),make_pair(0,1e9));
for (int i = 0;i<n1[x].size();i++){
if (n1[x][i]==p) continue;
dfs(n1[x][i],x);
pair<pair<int,int> , pair<int,int> > yay;
yay.first.first = min(dp[x].first.second+dp[n1[x][i]].first.first,dp[x].first.first+dp[n1[x][i]].second.first);
yay.first.second = min(dp[x].first.second+dp[n1[x][i]].second.first,dp[x].first.first+dp[n1[x][i]].first.first);
yay.second.first= min(dp[x].second.first+dp[n1[x][i]].second.second,dp[x].second.second+dp[n1[x][i]].first.second);
yay.second.second = min(dp[x].second.second+dp[n1[x][i]].second.second,dp[x].second.first+dp[n1[x][i]].first.second);
dp[x] = yay;
}
}
signed main(){
int n;
cin>>n;
n1.resize(n);
dp.resize(n);
n2.resize(n);
for (int i = 0;i<n-1;i++){
int a,b,c;
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];
dfs(0,-1);
int x = min(dp[0].second.second,dp[0].first.second);
if (x>=1e9) cout<<"impossible";
else cout<<x;
}
