#include <iostream>
#include <vector>
#include <algorithm>
#include <set>
#include <cmath>
#include <map>
#include <string>
#include <ios>
#include <iomanip>
#include <deque>
#include <queue>
#include <list>
#include <stack>
#define FASTIO ios_base::sync_with_stdio(0); cin.tie(NULL);
using ll = long long;
using namespace std;
vector<vector<ll>>gp;
vector<ll>v;
vector<vector<vector<ll>>>dp;
void dfs(ll u, ll p) {
ll cnt = 1e18, lox = 1e18, ans = 0, ans1 = 0;
for (ll& x : gp[u]) {
if (x != p) {
dfs(x, u);
dp[u][0][0] += min(dp[x][0][0], dp[x][1][0]);
dp[u][1][0] += min(dp[x][1][1], dp[x][0][1]);
dp[u][1][1] += min(dp[x][1][1], dp[x][0][1]);
dp[u][0][1] += min(dp[x][1][0], dp[x][0][0]);
if (min(dp[x][0][0], dp[x][1][0]) == dp[x][1][0]) ans++;
if (min(dp[x][0][1], dp[x][1][1]) == dp[x][1][1]) ans1++;
cnt = min(cnt, abs(dp[x][0][0] - dp[x][1][0]));
lox = min(lox, abs(dp[x][1][1] - dp[x][0][1]));
}
}
if (gp[u].size() == 1 && u != 0) {
if (v[u] == 0) {
dp[u][0][0] = 0;
dp[u][0][1] = 1e17;
dp[u][1][0] = 1e17;
dp[u][1][1] = 1;
}
else {
dp[u][0][0] = 1e17;
dp[u][0][1] = 0;
dp[u][1][0] = 1;
dp[u][1][1] = 1e17;
}
}
else {
dp[u][1][0]++;
dp[u][1][1]++;
if ((v[u] == 1 && ans % 2 == 0) || (v[u] == 0 && ans % 2 == 1)) {
dp[u][0][0] += cnt;
}
if ((v[u] == 1 && ans % 2 == 1) || (v[u] == 0 && ans % 2 == 0)) {
dp[u][0][1] += cnt;
}
if ((v[u] == 1 && ans1 % 2 == 0) || (v[u] == 0 && ans1 % 2 == 1)) {
dp[u][1][1] += lox;
}
if ((v[u] == 1 && ans1 % 2 == 1) || (v[u] == 0 && ans1 % 2 == 0)) {
dp[u][1][0] += lox;
}
}
long long inf = 1e18;
dp[u][0][0] = min(dp[u][0][0], inf);
dp[u][1][0] = min(dp[u][1][0], inf);
dp[u][1][1] = min(dp[u][1][1], inf);
dp[u][0][1] = min(dp[u][0][1], inf);
}
void solve() {
ll n; cin >> n;
v = vector<ll>(n);
gp = vector<vector<ll>>(n);
dp = vector<vector<vector<ll>>>(n, vector<vector<ll>>(2, vector<ll>(2)));
for (ll i = 0; i < n - 1; i++) {
ll u, v; cin >> u >> v;
--u, --v;
gp[u].push_back(v);
gp[v].push_back(u);
}
for (ll i = 0; i < n; i++) {
cin >> v[i];
}
dfs(0, -1);
if (min({ dp[0][0][0],dp[0][1][0] }) > n) {
cout << "impossible" << endl;
}
else cout << min({ dp[0][0][0],dp[0][1][0] });
}
signed main() {
FASTIO
ll t = 1;
// cin >> t;
while (t--) {
solve();
}
}
