#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
using namespace std;
#ifndef ONLINE_JUDGE
#define dbg(x) cerr << #x <<" "; print(x); cerr << endl;
#else
#define dbg(x)
#endif
#define sz(x) (int)x.size()
#define len(x) (int)x.length()
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define clr(x) (x).clear()
#define uniq(x) x.resize(unique(all(x)) - x.begin());
#define blt __builtin_popcount
#define pb push_back
#define popf pop_front
#define popb pop_back
#define ld long double
#define ll long long
void print(long long t) { cerr << t; }
void print(int t) { cerr << t; }
void print(string t) { cerr << t; }
void print(char t) { cerr << t; }
void print(double t) { cerr << t; }
void print(long double t) { cerr << t; }
void print(unsigned long long t) { cerr << t; }
template <class T, class V> void print(pair <T, V> p);
template <class T> void print(vector <T> v);
template <class T> void print(set <T> v);
template <class T, class V> void print(map <T, V> v);
template <class T> void print(multiset <T> v);
template <class T, class V> void print(T v[], V n) { cerr << "["; for (int i = 0; i < n; i++) { print(v[i]); cerr << " "; } cerr << "]"; }
template <class T, class V> void print(pair <T, V> p) { cerr << "{"; print(p.first); cerr << ","; print(p.second); cerr << "}"; }
template <class T> void print(vector <T> v) { cerr << "[ "; for (T i : v) { print(i); cerr << " "; } cerr << "]"; }
template <class T> void print(set <T> v) { cerr << "[ "; for (T i : v) { print(i); cerr << " "; } cerr << "]"; }
template <class T> void print(multiset <T> v) { cerr << "[ "; for (T i : v) { print(i); cerr << " "; } cerr << "]"; }
template <class T, class V> void print(map <T, V> v) { cerr << "[ "; for (auto i : v) { print(i); cerr << " "; } cerr << "]"; }
template <class T, class V> void print(unordered_map <T, V> v) { cerr << "[ "; for (auto i : v) { print(i); cerr << " "; } cerr << "]"; }
template <class T> void print(deque <T> v) { cerr << "[ "; for (T i : v) { print(i); cerr << " "; } cerr << "]"; }
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
#define nl '\n'
// for random generations
mt19937 myrand(chrono::steady_clock::now().time_since_epoch().count());
// mt19937 myrand(131);
// for grid problems
int dx[8] = { -1,0,1,0,1,-1,1,-1 };
int dy[8] = { 0,1,0,-1,1,1,-1,-1 };
// lowest / (1 << 17) >= 1e5 / (1 << 18) >= 2e5 / (1 << 21) >= 1e6
void fastIO() {
ios_base::sync_with_stdio(false);
cin.tie(nullptr); cout.tie(nullptr);
}
// file in/out
void setIO(string str = "") {
fastIO();
if (str == "input") {
freopen("input.txt", "r", stdin);
freopen("output.txt", "w", stdout);
}
else {
// freopen("skis.in", "r", stdin);
// freopen("skis.out", "w", stdout);
}
}
// Indexed Set
template <class T> using Tree = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
const int N = 2e5 + 10, inf = 1e8;
long long dp[N][2][2];
vector<int> adj[N];
int n, a[N];
void dfs(int node, int parent) {
dp[node][0][a[node]] = 0;
dp[node][1][a[node] ^ 1] = 1;
for(auto i: adj[node]) {
if(i == parent) continue;
dfs(i, node);
vector<vector<long long>> new_dp(2, vector<long long> (2, inf));
if(dp[i][0][0] != inf) {
new_dp[0][0] = min(new_dp[0][0], dp[node][0][0] + dp[i][0][0]);
new_dp[0][1] = min(new_dp[0][1], dp[node][0][1] + dp[i][0][0]);
}
if(dp[i][0][1] != inf) {
new_dp[1][0] = min(new_dp[1][0], dp[node][1][0] + dp[i][0][1]);
new_dp[1][1] = min(new_dp[1][1], dp[node][1][1] + dp[i][0][1]);
}
if(dp[i][1][0] != inf) {
new_dp[0][0] = min(new_dp[0][0], dp[node][0][1] + dp[i][1][0]);
new_dp[0][1] = min(new_dp[0][1], dp[node][0][0] + dp[i][1][0]);
}
if(dp[i][1][1] != inf) {
new_dp[1][0] = min(new_dp[1][0], dp[node][1][1] + dp[i][1][1]);
new_dp[1][1] = min(new_dp[1][1], dp[node][1][0] + dp[i][1][1]);
}
for(int j = 0; j < 2; j++) {
for(int k = 0; k < 2; k++) {
dp[node][j][k] = new_dp[j][k];
}
}
}
}
void solve_() {
cin >> n;
for(int i = 1; i <= n - 1; i++) {
int x, y; cin >> x >> y;
adj[x].push_back(y);
adj[y].push_back(x);
}
for(int i = 1; i <= n; i++) {
for(int j = 0; j <= 1; j++) {
for(int k = 0; k <= 1; k++) {
dp[i][j][k] = inf;
}
}
}
for(int i = 1; i <= n; i++) {
cin >> a[i];
}
dfs(1, 0);
long long ans = min(dp[1][0][0], dp[1][1][0]);
if(ans < inf) {
cout << ans << '\n';
} else {
cout << "impossible" << '\n';
}
}
int main() {
setIO();
auto solve = [&](int test_case)-> void {
for (int i = 1; i <= test_case; i++) {
solve_();
}
};
int test_cases = 1;
// cin >> test_cases;
solve(test_cases);
return 0;
}
