#include "bits/stdc++.h"
using namespace std;
void abc() {cout << endl;}
template <typename T, typename ...U> void abc(T a, U ...b) {
cout << a << ' ', abc(b...);
}
template <typename T> void printv(T l, T r) {
while (l != r) cout << *l << " \n"[++l == r];
}
template <typename A, typename B> istream& operator >> (istream& o, pair<A, B> &a) {
return o >> a.first >> a.second;
}
template <typename A, typename B> ostream& operator << (ostream& o, pair<A, B> a) {
return o << '(' << a.first << ", " << a.second << ')';
}
template <typename T> ostream& operator << (ostream& o, vector<T> a) {
bool is = false;
for (T i : a) {o << (is ? ' ' : '{'), is = true, o << i;}
return o << '}';
}
#ifdef local
#define test(args...) abc("[" + string(#args) + "]", args)
#else
#define test(args...) void(0)
#endif
using ll = long long;
vector<int> adj[100005];
int status[100005];
pair<int, ll> cost[100005][2]; // 0 off, 1 on
void dfs(int node, int par = -1) {
int child = 0;
ll noct = 0, nocost = 0, onct = 0, oncost = 0;
for (auto nxt : adj[node]) {
if (nxt == par) continue;
dfs(nxt, node);
child++;
noct += cost[nxt][0].first;
nocost += cost[nxt][0].second;
onct += cost[nxt][1].first;
oncost += cost[nxt][1].second;
}
if (child == 0) {
cost[node][status[node]] = {0, 0};
cost[node][1 - status[node]] = {1, 1};
return;
}
// dont place on this block
{
int curstat = status[node] ^ (noct % 2);
if (curstat == 0) {
cost[node][0].first = 0;
cost[node][0].second = nocost;
} else {
cost[node][1].first = 0;
cost[node][1].second = nocost;
}
}
// place on this block
{
int curstat = status[node] ^ (onct % 2);
if (curstat == 0) {
cost[node][1].first = 1;
cost[node][1].second = oncost + 1;
} else {
cost[node][0].first = 1;
cost[node][0].second = oncost + 1;
}
}
//
// if (status[node] == 0) {
// int curstat = status[node] ^ (noct % 2);
//
// if (curstat == 1) {
// // impossible
// cost[node][0].second = 1e9;
// } else {
// cost[node][0].first = 0;
// cost[node][0].second = nocost;
// }
// } else {
// int curstat = status[node] ^ (onct % 2);
//
// if (curstat == 0) {
// // impossible
// cost[node][1].second = 1e9;
// } else {
// cost[node][0].first = 0;
// cost[node][0].second = nocost;
// }
// }
//
// int curstat = status[node] ^ (noct % 2);
// int req = bool(curstat != 1);
// cost[node][1].first = req;
// cost[node][1].second = nocost + req;
//
// curstat = status[node] ^ (onct % 2);
// req = bool(curstat != 0);
// cost[node][0].first = req;
// cost[node][0].second = oncost + req;
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
// freopen("", "r", stdin);
// freopen("", "w", stdout);
int n; cin >> n;
for (int i = 0; i < n-1; i++) {
int a, b; cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for (int i = 1; i <= n; i++) cin >> status[i];
fill(&cost[0][0], &cost[0][0] + sizeof(cost) / sizeof(cost[0][0]), pair<int, ll> {0, 1e9});
dfs(1);
if (cost[1][0].second >= 1e9) cout << "impossible" << endl;
else cout << cost[1][0].second << endl;
}
