#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <chrono>
#include <ctime>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <limits>
#include <iomanip>
#include <unordered_set>
#include <unordered_map>
#include <fstream>
#include <functional>
#include <random>
#include <cassert>
using namespace std;
typedef long long ll;
typedef long double ld;
#define ff first
#define ss second
ll ttt;
const ll INF = 1e18;
const ll MOD = 1e9 + 7;
const ll N = 200007;
ll n, m;
vector<int>g[N];
bool vl[N];
ll dp[N][2][2];
void dfs(int v, int par = -1) {
dp[v][0][0] = dp[v][1][0] = dp[v][1][1] = dp[v][0][1] = MOD;
bool gnac = false;
for (int to : g[v]) {
if (to != par) {
dfs(to, v);
gnac = true;
}
}
//cout << v << ": ";
if (!gnac) {
dp[v][vl[v]][0] = 0;
dp[v][!vl[v]][1] = 1;
}
else {
ll mx = 0;
int cnt = 0;
for (int to : g[v]) {
if (to != par) {
if (dp[to][0][0] <= dp[to][0][1]) {
mx += dp[to][0][0];
}
else {
mx += dp[to][0][1];
cnt++;
}
}
}
cnt %= 2;
//cout << "cnt = " << cnt << endl;
//cout << "mx = " << mx << endl;
bool val = vl[v];
if (cnt) {
val = !val;
}
dp[v][val][0] = mx;
ll smx = MOD;
for (int to : g[v]) {
if (to != par) {
smx = min(smx, mx - min(dp[to][0][0], dp[to][0][1])
+ max(dp[to][0][0], dp[to][0][1]));
}
}
//cout << "smx = " << smx << endl;
dp[v][!val][0] = smx;
mx = 0;
cnt = 0;
val = vl[v];
smx = MOD;
for (int to : g[v]) {
if (to != par) {
if (dp[to][1][0] <= dp[to][1][1]) {
mx += dp[to][1][0];
}
else {
mx += dp[to][1][1];
cnt++;
}
}
}
cnt %= 2;
if (cnt) {
val = !val;
}
dp[v][!val][1] = mx + 1;
for (int to : g[v]) {
if (to != par) {
smx = min(smx, mx - min(dp[to][1][0], dp[to][1][1])
+ max(dp[to][1][0], dp[to][1][1]));
}
}
dp[v][val][1] = smx + 1;
}
//cout << dp[v][0][0] << " " << dp[v][0][1] << " "
// << dp[v][1][0] << " " << dp[v][1][1] << endl;
}
int main() {
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> n;
for (int i = 0; i < n - 1; i++) {
int x, y;
cin >> x >> y;
g[x].push_back(y);
g[y].push_back(x);
}
for (int i = 1; i <= n; i++) {
cin >> vl[i];
}
dfs(1);
ll ans = min(dp[1][0][0], dp[1][0][1]);
cout << (ans >= MOD ? "impossible" : to_string(ans)) << endl;
}
