#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <iomanip>
#include <array>
#include <string>
#include <algorithm>
#include <cmath>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <vector>
#include <stack>
#include <queue>
#include <deque>
#include <bitset>
#include <list>
#include <iterator>
#include <numeric>
#include <complex>
#include <utility>
#include <random>
#include <cassert>
#include <fstream>
using namespace std;
mt19937 rnd(7069);
/* -------------------- Typedefs -------------------- */
typedef int itn;
typedef long long ll;
typedef unsigned long long ull;
typedef double db;
typedef float fl;
typedef long double ld;
/* -------------------- Usings -------------------- */
using vi = vector<int>;
using vll = vector<ll>;
using mii = map<int, int>;
using mll = map<ll, ll>;
using pii = pair<int, int>;
using pll = pair<ll, ll>;
/* -------------------- Defines -------------------- */
#define ff first
#define ss second
#define pub push_back
#define pob pop_back
#define puf push_front
#define pof pop_front
#define mpr make_pair
#define yes cout<<"Yes\n"
#define no cout<<"No\n"
#define all(x) (x).begin(), (x).end()
#define USACO freopen("feast.in", "r", stdin); freopen("feast.out", "w", stdout);
/* -------------------- Constants -------------------- */
const int dx[8] = { -1, 0, 1, 0, -1, -1, 1, 1 };
const int dy[8] = { 0, -1, 0, 1, -1, 1, -1, 1 };
const int MAX = int(1e9 + 5);
const ll MAXL = ll(1e18) + 5ll;
const ll MOD = ll(1000000007);
const ll MOD2 = ll(998244353);
/* -------------------- Functions -------------------- */
void fastio() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
}
void precision(int x) {
cout.setf(ios::fixed | ios::showpoint);
cout.precision(x);
}
ll gcd(ll a, ll b) {
if (a == 0 || b == 0) return(max(a, b));
while (b) {
a %= b;
swap(a, b);
}
return a;
}
ll lcm(ll a, ll b) {
return a / gcd(a, b) * b;
}
ll range_sum(ll a, ll b) {
if (a > b) return 0ll;
ll dif = a - 1, cnt = b - a + 1;
ll ans = ((b - a + 1) * (b - a + 2)) / 2;
ans += ((b - a + 1) * dif);
return ans;
}
string dec_to_bin(ll a) {
string s = "";
for (ll i = a; i > 0; ) {
ll k = i % 2;
i /= 2;
char c = k + 48;
s += c;
}
if (a == 0) {
s = "0";
}
reverse(all(s));
return s;
}
ll bin_to_dec(string s) {
ll num = 0;
for (int i = 0; i < s.size(); i++) {
num *= 2ll;
num += (s[i] - '0');
}
return num;
}
ll factorial_by_mod(ll n, ll mod) {
ll ans = 1;
ll num;
for (ll i = 1; i <= n; ++i) {
num = i % mod;
ans *= num;
ans %= mod;
}
return ans;
}
int isPrime(ll a) {
if (a == 1) return 0;
for (ll i = 2; i * i <= a; i++) {
if (a % i == 0) return 0;
}
return 1;
}
ll binpow(ll a, ll b) {
if (!a) return 0;
ll ans = 1;
while (b) {
if (b & 1) {
ans *= a;
}
b >>= 1;
a *= a;
}
return ans;
}
ll binpow_by_mod(ll a, ll b, ll mod) {
if (!a) return 0;
ll ans = 1;
while (b) {
if (b & 1) {
ans *= a;
ans %= mod;
}
b >>= 1;
a *= a;
a %= mod;
}
return ans;
}
/* -------------------- Solution -------------------- */
const int N = 100005, M = 5;
int a[N], dp[N][M][M], check[N][M][M];
vi g[N];
int n;
void dfs(int u = 1, int p = 1) {
bool f = true;
for (auto to : g[u]) {
if (to == p) continue;
f = false;
dfs(to, u);
}
if (f) {
if (a[u] == 1) {
check[u][1][0] = 1;
check[u][2][1] = 1;
dp[u][1][0] = 0;
dp[u][2][1] = 1;
}
else {
check[u][2][0] = 1;
check[u][1][1] = 1;
dp[u][2][0] = 0;
dp[u][1][1] = 1;
}
return;
}
int mn = MAX;
int cur = a[u], cnt = 0;
f = true;
for (auto to : g[u]) {
if (to == p) continue;
if (check[to][2][0] && check[to][2][1]) {
if (dp[to][2][0] < dp[to][2][1]) {
cnt += dp[to][2][0];
}
else {
cnt += dp[to][2][1];
cur ^= 1;
}
mn = min(mn, abs(dp[to][2][0] - dp[to][2][1]));
}
else if (check[to][2][0]) {
cnt += dp[to][2][0];
}
else if (check[to][2][1]) {
cnt += dp[to][2][1];
cur ^= 1;
}
else {
f = false;
}
}
if (f) {
if (cur == 1) {
check[u][1][0] = 1;
dp[u][1][0] = cnt;
}
else {
check[u][2][0] = 1;
dp[u][2][0] = cnt;
if (mn != MAX) {
cnt += mn;
check[u][1][0] = 1;
dp[u][1][0] = cnt;
}
}
}
mn = MAX;
cur = a[u], cnt = 0;
f = true;
for (auto to : g[u]) {
if (to == p) continue;
if (check[to][1][0] && check[to][1][1]) {
if (dp[to][1][0] < dp[to][1][1]) {
cnt += dp[to][1][0];
}
else {
cnt += dp[to][1][1];
cur ^= 1;
}
mn = min(mn, abs(dp[to][1][0] - dp[to][1][1]));
}
else if (check[to][1][0]) {
cnt += dp[to][1][0];
}
else if (check[to][1][1]) {
cnt += dp[to][1][1];
cur ^= 1;
}
else {
f = false;
}
}
if (f) {
if (cur == 1) {
check[u][2][1] = 1;
dp[u][2][1] = min(dp[u][2][1], cnt + 1);
}
else {
if (mn != MAX) {
check[u][1][1] = 1;
cnt += mn;
dp[u][1][1] = min(dp[u][1][1], cnt);
}
}
}
}
void slv() {
cin >> n;
for (int i = 0; i < n - 1; ++i) {
int x, y;
cin >> x >> y;
g[x].pub(y);
g[y].pub(x);
}
for (int i = 1; i <= n; ++i) {
cin >> a[i];
dp[i][1][0] = dp[i][1][1] = dp[i][2][0] = dp[i][2][1] = MAX;
}
dfs();
int ans = min(dp[1][2][0], dp[1][2][1]);
if (ans == MAX) cout << "impossible";
else cout << ans;
}
void cs() {
int tstc = 1;
//cin >> tstc;
while (tstc--) {
slv();
}
}
void precalc() {
return;
}
int main() {
fastio();
precalc();
//precision(0);
cs();
return 0;
}
