#include <bits/stdc++.h>
/*
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
*/
using namespace std;
// using namespace __gnu_pbds;
using namespace chrono;
// mt19937 rng((int) std::chrono::steady_clock::now().time_since_epoch().count());
/*
template <class T> using ordered_set = tree <T, null_type, less <T>, rb_tree_tag, tree_order_statistics_node_update>;
*/
//***************** CONSTANTS *****************
const int MXN = 100000;
const int INF = 0x3f3f3f3f;
//***************** GLOBAL VARIABLES *****************
int dp[MXN][2][2], on[MXN];
vector<int> g[MXN];
//***************** AUXILIARY STRUCTS *****************
//***************** MAIN BODY *****************
void dfs(int u, int p){
int temp[2][2][2];
memset(temp, INF, sizeof temp);
temp[0][0][0] = temp[0][1][0] = 0;
for(int v : g[u]) if(v != p){
dfs(v, u);
memset(temp[1], INF, sizeof temp[1]);
for(int i = 0; i < 2; i++)
for(int j = 0; j < 2; j++)
for(int k = 0; k < 2; k++)
temp[1][i][j^k] = min(temp[1][i][j^k], temp[0][i][j] + dp[v][i][k]);
swap(temp[0], temp[1]);
}
dp[u][0][0] = temp[0][0][on[u]];
dp[u][0][1] = temp[0][1][on[u]^1] + 1;
dp[u][1][0] = temp[0][0][on[u]^1];
dp[u][1][1] = temp[0][1][on[u]] + 1;
}
void solve(){
int N;
cin >> N;
for(int i = 0; i < N - 1; i++){
int u, v;
cin >> u >> v;
--u, --v;
g[u].push_back(v);
g[v].push_back(u);
}
for(int i = 0; i < N; i++)
cin >> on[i];
memset(dp, INF, sizeof dp);
dfs(0, 0);
int ans = min(dp[0][0][0], dp[0][0][1]);
if(ans > N) cout << "impossible\n";
else cout << ans << '\n';
}
/*
dp[N][2][2] -> dp[u][i][j] -> currently considering the uth node in state i and press j
temp[c][2][2] -> tmp[v][i][j] -> vth child, is u toggled, j -> parity of selection
*/
//***************** *****************
int32_t main(){
ios_base::sync_with_stdio(NULL);
cin.tie(NULL);
#ifdef LOCAL
auto begin = high_resolution_clock::now();
#endif
int tc = 1;
// cin >> tc;
for (int t = 0; t < tc; t++)
solve();
#ifdef LOCAL
auto end = high_resolution_clock::now();
cout << fixed << setprecision(4);
cout << "Execution Time: " << duration_cast<duration<double>>(end - begin).count() << "seconds" << endl;
#endif
return 0;
}
/*
If code gives a WA, check for the following :
1. I/O format
2. Are you clearing all global variables in between tests if multitests are a thing
3. Can you definitively prove the logic
*/
