// Hallelujah, praise the one who set me free
// Hallelujah, death has lost its grip on me
// You have broken every chain, There's salvation in your name
// Jesus Christ, my living hope
#include <bits/stdc++.h>
using namespace std;
#define REP(i, s, e) for (int i = (s); i < (e); i++)
#define RREP(i, s, e) for (int i = (s); i >= (e); i--)
template <class T>
inline bool mnto(T& a, T b) {return a > b ? a = b, 1 : 0;}
template <class T>
inline bool mxto(T& a, T b) {return a < b ? a = b, 1: 0;}
typedef long long ll;
typedef long double ld;
#define FI first
#define SE second
typedef pair<int, int> ii;
typedef pair<ll, ll> pll;
typedef tuple<int, int, int> iii;
#define ALL(_a) _a.begin(), _a.end()
#define SZ(_a) (int) _a.size()
#define pb push_back
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<ii> vii;
typedef vector<iii> viii;
#ifndef DEBUG
#define cerr if (0) cerr
#endif
const int INF = 1000000005;
const ll LINF = 1000000000000000005ll;
const int MAXN = 100005;
int n;
vi adj[MAXN];
bool a[MAXN];
int dp[MAXN][2][2];
void dfs(int u, int p) {
bool rt = 1;
for (int v : adj[u]) {
if (v == p) {
continue;
}
rt = 0;
dfs(v, u);
}
if (rt) {
REP (x, 0, 2) {
REP (y, 0, 2) {
dp[u][x][y] = INF;
}
}
REP (x, 0, 2) {
dp[u][x][a[u] ^ x] = x;
}
return;
}
REP (x, 0, 2) { // x = whether self is pressed
int y = a[u] ^ x;
int sm = x;
for (int v : adj[u]) {
if (v == p) {
continue;
}
if (dp[v][0][x] < dp[v][1][x]) {
sm += dp[v][0][x];
mnto(sm, INF);
} else {
sm += dp[v][1][x];
mnto(sm, INF);
y ^= 1;
}
}
dp[u][x][y] = sm;
int mn = INF;
for (int v : adj[u]) {
if (v == p) {
continue;
}
mnto(mn, abs(dp[v][0][x] - dp[v][1][x]));
}
dp[u][x][y ^ 1] = min(sm + mn, INF);
}
}
int main() {
#ifndef DEBUG
ios::sync_with_stdio(0), cin.tie(0);
#endif
cin >> n;
REP (i, 1, n) {
int u, v; cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
REP (i, 1, n + 1) {
cin >> a[i];
}
dfs(1, -1);
int ans = INF;
REP (x, 0, 2) {
mnto(ans, dp[1][x][0]);
}
if (ans == INF) {
cout << "impossible\n";
} else {
cout << ans << '\n';
}
return 0;
}
