#include <bits/stdc++.h>
#define se second
#define fs first
#define pb push_back
#define ll long long
#define ii pair<ll,ll>
#define ld long double
#define SZ(v) (int)v.size()
#define ALL(v) v.begin(), v.end()
#define bit(msk, i) ((msk >> i) & 1)
#define iter(id, v) for(auto id : v)
#define rep(i,m,n) for(int i=(m); i<=(n); i++)
#define reb(i,m,n) for(int i=(m); i>=(n); i--)
using namespace std;
mt19937_64 rd(chrono :: steady_clock :: now().time_since_epoch().count());
ll Rand(ll l, ll r) { return uniform_int_distribution<ll> (l, r)(rd); }
const int N = 3e5 + 7;
const ll Mod = 1e9 + 7;
const int szBL = 916;
const ll INF = 1e12;
const int BASE = 137;
int n, Q;
int a[N];
ll dp[N][2][2];
vector<int> ke[N];
void dfs (int u, int p) {
iter (&v, ke[u]) {
if (v == p) continue;
dfs (v, u);
// rep (i, 0, 1) rep (j, 0, 1) cout<<v<<","<<i<<","<<j<<": "<<dp[v][i][j] <<"\n";
}
if (SZ(ke[u]) == 1 && p != 0) {
dp[u][a[u]][0] = 0;
dp[u][a[u] ^ 1][1] = 1;
return;
}
static vector<ll> vec; vector<ll> ().swap(vec);
ll tot = 0;
iter(&v, ke[u]) if (v != p) {
tot += dp[v][0][0];
if (dp[v][0][1] < INF) vec.push_back(-dp[v][0][0] + dp[v][0][1]);
else vec.push_back(INF);
}
sort (ALL(vec));
dp[u][a[u]][0] = min(dp[u][a[u]][0], tot);
rep (i, 0, SZ(vec) - 1) {
tot += vec[i];
if ((i + 1) % 2 == a[u]) dp[u][0][0] = min (dp[u][0][0], tot);
else dp[u][1][0] = min(dp[u][1][0], tot);
}
tot = 0;
vector<ll> ().swap(vec);
iter(&v, ke[u]) if (v != p) {
tot += dp[v][1][0];
if (dp[v][1][1] < INF) vec.push_back(-dp[v][1][0] + dp[v][1][1]);
else vec.push_back(INF);
}
dp[u][a[u] ^ 1][1] = min(dp[u][a[u] ^ 1][1], tot + 1);
// if (u == 3) cout << SZ(vec) <<" hi\n";
sort (ALL(vec));
rep (i, 0, SZ(vec) - 1) {
tot += vec[i];
if ((i + 1) % 2 == a[u]) dp[u][1][1] = min(dp[u][1][1], tot + 1);
else {
dp[u][0][1] = min(dp[u][0][1], tot + 1);
// if (u == 1) cout << dp[u][0][1] <<" hji\n";
}
}
}
void solution() {
cin >> n;
rep (i, 1, n - 1 ) {
int u, v;
cin >> u >> v;
ke[u].push_back(v);
ke[v].push_back(u);
}
rep (i, 1, n) {
cin >> a[i];
}
rep (i, 1, n) rep (j, 0, 1) rep (k, 0, 1) dp[i][j][k] = INF;
dfs(1, 0);
ll res = min(dp[1][0][1], dp[1][0][0]);
if (res < INF) cout << res <<"\n";
else cout <<"impossible\n";
}
#define file(name) freopen(name".inp", "r", stdin); freopen(name".out", "w", stdout);
int main () {
// file("c");
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
ll num_Test = 1;
// cin >> num_Test;
while(num_Test--)
solution();
}
/*
18 3
2 5
6 21
13 19
9 17
14 17
19 20
2 16
2 10
9 14
19 20
14 16
1 3
17 19
14 21
18 19
4 7
5 12
1 13
*/
