#include <bits/stdc++.h>
using namespace std;
const int N = 100005;
const int INF = 0x3f3f3f3f;
int ckmin(int& x, int y){return x>y?x=y:x;}
int n;
vector<int> adj[N];
int arr[N];
int dp[N][2][2]; // vertex, value of the current node, whether this node is chosen
void dfs(int v, int p){
int tmp[2][2][2];
memset(tmp, INF, sizeof tmp);
tmp[0][0][0] = tmp[0][1][0] = 0;
for(auto u : adj[v]){
if(u == p) continue;
dfs(u, v);
memset(tmp[1], INF, sizeof tmp[1]);
for(int a = 0; a<2; a++) // dp & tmp
for(int b = 0; b<2; b++) // dp
for(int c = 0; c<2; c++) // tmp
ckmin(tmp[1][a][b^c], dp[u][a][b]+tmp[0][a][c]);
swap(tmp[0], tmp[1]);
}
dp[v][0][0] = tmp[0][0][arr[v]];
dp[v][0][1] = tmp[0][1][1^arr[v]]+1;
dp[v][1][0] = tmp[0][0][1^arr[v]];
dp[v][1][1] = tmp[0][1][arr[v]]+1;
}
int main(){
// freopen("a.in", "r", stdin);
ios_base::sync_with_stdio(false);
cin.tie(0);
cin >> n;
for(int i = 0; i<n-1; i++){
int a, b; cin >> a >> b; --a; --b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for(int i = 0; i<n; i++)
cin >> arr[i];
memset(dp, INF, sizeof dp);
dfs(0, -1);
int ans = min(dp[0][0][0], dp[0][0][1]);
if(ans == INF) cout << "impossible" << '\n';
else cout << ans << '\n';
}
