#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define AquA cin.tie(0);ios_base::sync_with_stdio(0);
#define fs first
#define sc second
#define p_q priority_queue
#define int long long
using namespace std;
const int inf=1e9;
struct no{
vector<int> ch;
int dp[2][2]={inf,inf,inf,inf};
int t=0;
};
vector<no> v;
void dfs(int r,int f){
for(auto h:v[r].ch){
if(h!=f){
dfs(h,r);
}
}
for(int j=0;j<2;j++){
int a=0,b=inf;
for(auto h:v[r].ch){
if(h!=f){
int c=min(a+v[h].dp[j][0],b+v[h].dp[j][1]);
int d=min(a+v[h].dp[j][1],b+v[h].dp[j][0]);
a=c;
b=d;
}
}
v[r].dp[v[r].t^j][j]=a+j;
v[r].dp[v[r].t^j^1][j]=b+j;
}
}
signed main(){
AquA;
int n;
cin >> n;
v.resize(n);
for(int i=1;i<n;i++){
int a,b;
cin >> a >> b;
a--;
b--;
v[a].ch.push_back(b);
v[b].ch.push_back(a);
}
for(int i=0;i<n;i++){
cin >> v[i].t;
}
dfs(0,0);
int mn=min(v[0].dp[0][0],v[0].dp[0][1]);
if(mn>5e8){
cout << "impossible" << "\n";
return 0;
}
cout << mn << "\n";
return 0;
}
