#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
vector<vector<int>> g;
vector<int> a;
vector<array<int, 4>> dp;
void dfs(int v, int la){
if (g[v].size()<2 and la!=-1){
if (a[v]==0){
dp[v]={1, (int)1e9, (int)1e9, 0};
} else {
dp[v]={(int)1e9, 0, 1, (int)1e9};
}
return;
}
array<int, 4> ar={0, 0, 0, 0};
for (auto u: g[v]){
if (u!=la){
dfs(u, v);
if (ar[0]==1e9 or dp[u][0]==1e9){
ar[0]=1e9;
} else {
ar[0]+=dp[u][0];
}
if (ar[1]==1e9 or dp[u][1]==1e9){
ar[1]=1e9;
} else {
ar[1]+=dp[u][1];
}
if (ar[2]==1e9 or dp[u][2]==1e9){
ar[2]=1e9;
} else {
ar[2]+=dp[u][2];
}
if (ar[3]==1e9 or dp[u][3]==1e9){
ar[3]=1e9;
} else {
ar[3]+=dp[u][3];
}
}
}
if (ar[0]!=1e9){
ar[0]++;
}
if (ar[1]!=1e9){
ar[1]++;
}
if (ar[2]!=1e9){
ar[2]++;
}
if (ar[3]!=1e9){
ar[3]++;
}
if (a[v]==1){
dp[v]={ar[0], ar[3], ar[1], ar[2]};
} else {
dp[v]={ar[1], ar[2], ar[0], ar[3]};
}
return;
}
int main(){
cin.tie(0); ios::sync_with_stdio(NULL);
int n;
cin>>n;
g.resize(n);
a.resize(n);
dp.resize(n);
for (int i=0; i<n-1; i++){
int a, b;
cin>>a>>b;
a--;
b--;
g[a].push_back(b);
g[b].push_back(a);
}
for (int i=0; i<n; i++){
cin>>a[i];
}
dfs(0, -1);
if (dp[0][2]==1e9 and dp[0][3]==1e9){
cout<<"impossible"<<"\n";
} else {
cout<<min(dp[0][2], dp[0][3])<<"\n";
}
}
