#include<bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using ull = unsigned long long;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
using pld = pair<ld, ld>;
#define fi first
#define se second
#define left BAO
#define right ANH
#define pb push_back
#define pf push_front
#define mp make_pair
#define ins insert
#define btpc __builtin_popcount
#define btclz __builtin_clz
#define sz(x) (int)(x.size());
#define all(x) x.begin(), x.end()
#define debug(...) " [" << #__VA_ARGS__ ": " << (__VA_ARGS__) << "] "
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int d4x[4] = {1, 0, -1, 0}; int d4y[4] = {0, 1, 0, -1};
int d8x[8] = {0, 1, 1, 1, 0, -1, -1, -1};
int d8y[8] = {1, 1, 0, -1, -1, -1, 0, 1};
template<class X, class Y>
bool minimize(X &x, const Y &y) {
if (x > y)
{
x = y;
return true;
}
return false;
}
template<class X, class Y>
bool maximize(X &x, const Y &y) {
if (x < y)
{
x = y;
return true;
}
return false;
}
const int MOD = 1e9 + 7; //998244353
template<class X, class Y>
void add(X &x, const Y &y) {
x = (x + y);
if(x >= MOD) x -= MOD;
}
template<class X, class Y>
void sub(X &x, const Y &y) {
x = (x - y);
if(x < 0) x += MOD;
}
/* Author : Le Ngoc Bao Anh, 12A5, LQD High School for Gifted Student*/
const ll INF = 1e9;
const int N = 1e5 + 10;
ll dp[N][2][2];
ll f[N][2][2], nf[2][2];
vector<int> adj[N];
int state[N];
void dfs(int u, int par) {
//f(press, color)
int child = 0;
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 2; j++) {
f[u][i][j] = dp[u][i][j] = INF;
}
}
f[u][0][0] = f[u][0][1] = 0;
for(auto v : adj[u]) {
if(v != par) {
dfs(v, u);
child = true;
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 2; j++) {
nf[i][j] = INF;
}
}
for(int j = 0; j < 2; j++) {
for(int i = 0; i < 2; i++) {
for(int x = 0; x < 2; x++) {
minimize(nf[i ^ x][j], f[u][i][j] + dp[v][x][j]);
}
}
}
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 2; j++) {
f[u][i][j] = nf[i][j];
}
}
}
}
if(!child) {
for(int i = 0; i < 2; i++) {
dp[u][i][state[u] ^ i] = 1;
}
return;
}
for(int i = 0; i < 2; i++) {
for(int j = 0; j < 2; j++) {
minimize(dp[u][j][state[u] ^ i ^ j], f[u][i][j] + j);
}
}
}
void BaoJiaoPisu() {
int n; cin >> n;
for(int i = 1; i < n; i++) {
int u, v; cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
for(int i = 1; i <= n; i++) cin >> state[i];
dfs(1, 0);
ll ans = min(dp[1][0][0], dp[1][1][0]);
if(ans >= INF) cout << "impossible";
else cout << ans;
}
int main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int tc = 1, ddd = 0;
// cin >> tc;
while(tc--) {
//ddd++;
//cout << "Case #" << ddd << ": ";
BaoJiaoPisu();
}
}
