#include <bits/stdc++.h>
using namespace std;
#define int long long
int n;
const int mxn = 100005;
vector<int> adj[mxn];
int a[mxn];
int dp[2][2][mxn];//dp[used or not][node i is lit up or not][node i] = min cost to reach said state
void dfs(int u, int p){
int c = 0;
for(int nxt: adj[u]){
if(nxt==p)continue;
dfs(nxt,u);
c++;
}
if(c==0){
if(a[u]==0){
dp[0][0][u] = 0;
dp[1][1][u] = 1;
dp[0][1][u] = dp[1][0][u] = (int)1e9;
}
else{
dp[0][0][u] = dp[1][1][u] = (int)1e9;
dp[0][1][u] = 0;
dp[1][0][u] = 1;
}
}
else{
vector<int>nodes;
for(int nxt: adj[u]){
if(nxt==p)continue;
nodes.push_back(nxt);
}
dp[0][0][u] = dp[0][1][u] = dp[1][0][u] = dp[1][1][u] = (int)1e9;
if(true){
int sum = 0;
vector<int>arr;
for(int i = 0; i<c; i++){
int x = dp[0][0][nodes[i]];
int y = dp[1][0][nodes[i]];
arr.push_back(y-x);
sum+=x;
}
int cur = a[u]^1;
sort(arr.begin(),arr.end());
dp[1][cur][u] = min(dp[1][cur][u],sum+1);
for(int i = 0; i<c; i++){
cur^=1;
sum+=arr[i];
dp[1][cur][u] = min(dp[1][cur][u],sum+1);
}
}
if(true){
int sum = 0;
vector<int>arr;
for(int i = 0; i<c; i++){
int x = dp[0][1][nodes[i]];
int y = dp[1][1][nodes[i]];
arr.push_back(y-x);
sum+=x;
}
int cur = a[u];
sort(arr.begin(),arr.end());
dp[0][cur][u] = min(dp[0][cur][u],sum);
for(int i = 0; i<c; i++){
cur^=1;
sum+=arr[i];
dp[0][cur][u] = min(dp[0][cur][u],sum);
}
}
/*
for(int i = 0; i<(1LL<<c); i++){
for(int j = 0; j<(1LL<<c); j++){
int used = a[u];
for(int a = 0; a<c; a++){
if(i&(1LL<<a)){
used^=1;
}
}
int sum = 0;
int tot = 0;
for(int a = 0; a<c; a++){
int x = (i&(1LL<<a));
int y = (j&(1LL<<a));
if(x>0)x = 1;
if(y>0)y = 1;
tot+=dp[x][y][nodes[a]];
}
for(int a = 0; a<c; a++){
if(j&(1LL<<a)){
sum++;
}
}
if(sum!=0&&sum!=c)continue;
if(sum==0){
dp[1][used^1][u] = min(dp[1][used^1][u],tot+1);
}
else{
dp[0][used][u] = min(dp[0][used][u],tot);
}
}
}
*/
}
}
signed main(){
cin.tie(nullptr)->sync_with_stdio(false);
cin >> n;
for(int i = 1; i<n; i++){
int x,y;
cin >> x >> y;
adj[x].push_back(y);
adj[y].push_back(x);
}
for(int i = 1; i<=n; i++){
cin >> a[i];
a[i]^=1;
}
dfs(1,0);
/*
for(int i = 1; i<=n; i++){
for(int a = 0; a<2; a++){
for(int b = 0; b<2; b++){
cout << dp[a][b][i] << " ";
}
}
cout << "\n";
}
*/
int v = min(dp[0][1][1],dp[1][1][1]);
if(v>(int)1e6)cout << "impossible\n";
else cout << v << "\n";
return 0;
}
