#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn5 = 1e6 + 10;
#define pb push_back
// dp[node][rangesh][khodesh zade shode ya na]
int dp[maxn5][2][2];
vector <int> adj[maxn5];
int ty[maxn5];
inline void dfs(int v, int par){
dp[v][0][0] = dp[v][0][1] = dp[v][1][0] = dp[v][1][1] = maxn5;
if(adj[v].size() == 1 && par != -1){
dp[v][ty[v]][0] = 0;
dp[v][ty[v]^1][1] = 1;
return;
}
// dp[v][hame 0/1][tedad zoj ya fard]
dp[v][0][0] = dp[v][1][0] = 0;
for(auto u : adj[v]) if(u != par){
dfs(u, v);
int x = dp[v][0][0], y = dp[v][0][1];
dp[v][0][0] = min({maxn5, x + dp[u][0][0], y + dp[u][0][1]});
dp[v][0][1] = min({maxn5, x + dp[u][0][1], y + dp[u][0][0]});
x = dp[v][1][0]; y = dp[v][1][1];
dp[v][1][0] = min({maxn5, x + dp[u][1][0], y + dp[u][1][1]});
dp[v][1][1] = min({maxn5, x + dp[u][1][1], y + dp[u][1][0]});
}
int a = dp[v][0][0], b = dp[v][0][1], c = dp[v][1][0], d = dp[v][1][1];
dp[v][ty[v]][0] = a;
dp[v][ty[v]][1] = d + 1;
dp[v][ty[v]^1][0] = b;
dp[v][ty[v]^1][1] = c + 1;
//cout << v << ' ' << par << ' ' << dp[v][0][0] << ' ' << dp[v][0][1] << ' ' << dp[v][1][0] << ' '<< dp[v][1][1] << endl;
return;
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
int n; cin >> n;
for(int i = 0; i < n - 1; i++){
int a, b; cin >> a >> b;
a--; b--;
adj[a].pb(b);
adj[b].pb(a);
}
for(int i = 0; i < n; i++)
cin >> ty[i];
dfs(0, -1);
ll ans = min(dp[0][0][0], dp[0][0][1]);
if(ans > n)
cout << "impossible\n";
else
cout << ans << endl;
}
