#include <bits/stdc++.h>
using namespace std;
#define file ""
#define mp make_pair
#define fi first
#define se second
#define all(x) x.begin(), x.end()
#define getbit(x, i) (((x) >> (i)) & 1)
#define bit(x) (1LL << (x))
#define popcount __builtin_popcountll
mt19937_64 rd(chrono::steady_clock::now().time_since_epoch().count());
int rand(int l, int r) {
return l + rd() % (r - l + 1);
}
const int N = 1e6 + 5;
const int mod = (int)1e9 + 7; // 998244353;
const int lg = 25; // lg + 1
const int oo = 1e9;
const long long ooo = 1e18;
template<class X, class Y> bool mini(X &a, Y b) {
return a > b ? (a = b, true) : false;
}
template<class X, class Y> bool maxi(X &a, Y b) {
return a < b ? (a = b, true) : false;
}
void add(int &a, int b) {
a += b;
if (a >= mod) a -= mod;
if (a < 0) a += mod;
}
int n;
vector<int> adj[N];
int state[N];
int f[N][2][2];
/*
f[u][i][j] : minimum number of moves to turn off subtree u
i = 0/1 if we toggle p
j = 0/1 if we toggle u
*/
void dfs(int u, int p) {
for (int i = 0; i < 2; ++i) {
f[u][i][state[u] ^ i] = i;
f[u][i][state[u] ^ i ^ 1] = oo;
}
for (int v : adj[u]) if (v != p) {
dfs(v, u);
for (int i = 0; i < 2; ++i) {
int even = f[u][i][0];
int odd = f[u][i][1];
f[u][i][0] = min({oo, even + f[v][0][i], odd + f[v][1][i]});
f[u][i][1] = min({oo, even + f[v][1][i], odd + f[v][0][i]});
}
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
// freopen(file".inp", "r", stdin);
// freopen(file".out", "w", stdout);
cin >> n;
for (int i = 1; i < n; ++i) {
int u, v;
cin >> u >> v;
adj[u].push_back(v);
adj[v].push_back(u);
}
for (int i = 1; i <= n; ++i) cin >> state[i];
dfs(1, 0);
int ans = min(f[1][0][0], f[1][1][0]);
if (ans == oo) return cout << "impossible", 0;
cout << ans;
return 0;
}
/*
*/
