#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1e5+5;
const int INF = 1e5+10;
int dp[MAXN][2][2],temp[2][2],n;
bool state[MAXN];
vector<int> adj[MAXN];
//let dp[u][p][t] be the minimum number of moves to turn off node u and its subtree where p is true if the parent toggled their bulb and false otherwise, t is if we intend to toggle the current nod
void dfs(int node, int par){
for(int i = 0; i <= 1; i++){
dp[node][i][state[node] ^ i] = i;
dp[node][i][state[node] ^ i ^ 1] = INF;
}
for(int child:adj[node]){
if(child == par) continue;
dfs(child,node);
for(int i = 0; i <= 1; i++){
int odd = dp[node][i][1];
int even = dp[node][i][0];
dp[node][i][0] = min({INF, even + dp[child][0][i], odd + dp[child][1][i]});
dp[node][i][1] = min({INF, even + dp[child][1][i], odd + dp[child][0][i]});
}
}
}
int main() {
cin >> n;
for(int i = 0; i < n-1; 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 >> state[i];
}
dfs(1,-1);
int ans = min(dp[1][0][0],dp[1][1][0]);
if(ans >= INF){
cout << "impossible\n";
}
else{
cout << ans << "\n";
}
}
