#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;
int yaye(vector<int> a, vector<int> b){
vector<int> c;
for (int i = 0;i<a.size();i++){
c.push_back(a[i]-b[i]);
}
sort(c.begin(),c.end());
int c1 = 0;
int ans = 0;
while (c1!=c.size() and c[c1]<=0) {ans+=c[c1];c1++;}
if (c1%2==1){
if (c1!=c.size()) ans = min(ans+c[c1],ans-c[c1-1]);
else ans = ans-c[c1-1];
}
for (int i = 0;i<b.size();i++) ans+=b[i];
return ans;
}
int yayo(vector<int> a, vector<int> b){
vector<int> c;
for (int i = 0;i<a.size();i++){
c.push_back(a[i]-b[i]);
}
sort(c.begin(),c.end());
int c1 = 0;
int ans = 0;
while (c1!=c.size() and c[c1]<=0) {ans+=c[c1];c1++;}
if (c1%2==0){
if (c1!=c.size() and c1!=0) ans = min(ans+c[c1],ans-c[c1-1]);
else if (c1==c.size()) ans = ans-c[c1-1];
else ans=ans+c[c1];
}
for (int i = 0;i<b.size();i++) ans+=b[i];
return ans;
}
void dfs(int x, int p){
if (n1[x].size()==1 and p!=-1){
if (n2[x]==1){
dp[x]=make_pair(make_pair(1e9,0),make_pair(1,1e9));
}
else{
dp[x] = make_pair(make_pair(1,1e9),make_pair(1e9,0));
}
return;
}
vector<int> one;
vector<int> two;
vector<int> three;
vector<int> four;
for (int i = 0;i<n1[x].size();i++){
if (n1[x][i]==p) continue;
dfs(n1[x][i],x);
one.push_back(dp[n1[x][i]].first.first);
two.push_back(dp[n1[x][i]].first.second);
three.push_back(dp[n1[x][i]].second.first);
four.push_back(dp[n1[x][i]].second.second);
}
if (n2[x]==1){
dp[x] = make_pair(make_pair(yayo(one,two)+1,yaye(three,four)),make_pair(yaye(one,two)+1,yayo(three,four)));
}
else{
dp[x] = make_pair(make_pair(yaye(one,two)+1,yayo(three,four)),make_pair(yayo(one,two)+1,yaye(three,four)));
}
}
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.first,dp[0].second.second);
if (x>=1e9) cout<<"Impossible";
else cout<<x;
}
