#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
void chmin(int &x, const int &y){
x = min(x, y);
}
int dp[100005][2][2];
signed main(){
int n; cin >> n;
vector <vector<int>> adj(n);
for ( int i = 1; i < n; i++ ){
int u, v; cin >> u >> v;
--u, --v;
adj[u].push_back(v);
adj[v].push_back(u);
}
vector <int> a(n);
for ( auto &u: a ) cin >> u;
for ( int i = 0; i < n; i++ ){
for ( int j = 0; j < 2; j++ ){
dp[i][j][0] = dp[i][j][1] = n + 1;
}
}
auto dfs = [&](auto dfs, int u, int p) -> void{
vector <int> x(2, n + 1), y(2, n + 1);
x[0] = y[0] = 0;
for ( auto &v: adj[u] ){
if ( v != p ){
dfs(dfs, v, u);
vector <int> xx(2), yy(2);
for ( auto k: {0, 1} ){
xx[k] = min(dp[v][0][k] + x[0], dp[v][0][k ^ 1] + x[1]);
yy[k] = min(dp[v][1][k] + y[0], dp[v][1][k ^ 1] + y[1]);
}
swap(xx, x), swap(yy, y);
}
}
for ( auto k: {0, 1} ){
chmin(dp[u][a[u] ^ k][0], x[k]);
chmin(dp[u][a[u] ^ k ^ 1][1], y[k] + 1);
}
};
dfs(dfs, 0, -1);
int ans = min(dp[0][0][1], dp[0][0][0]);
if ( ans == n + 1 ) return cout << "impossible\n", 0;
cout << ans << '\n';
}
