#include <iostream>
#include <vector>
using namespace std;
#define vi vector<int>
#define vvi vector<vi>
vvi adj;
vi on;
vvi dfs(int v, int p) {
vvi ans = vvi(2, vi(2, -1));
bool start = false;
for (int u : adj[v]) {
if (u == p) continue;
vvi res = dfs(u, v);
if (!start) {
ans = res, start = true;
continue;
}
vvi nxt = vvi(2, vi(2, -1));
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j)
if (min(ans[i][j], res[i][j]) >= 0)
nxt[i][0] = max(nxt[i][0], ans[i][j] + res[i][j]);
for (int j = 0; j < 2; ++j)
if (min(ans[i][j], res[i][1 - j]) >= 0)
nxt[i][1] = max(nxt[i][1], ans[i][j] + res[i][1 - j]);
}
ans = nxt;
}
if (!start) ans[0][0] = ans[1][0] = 0;
vvi copy = ans;
if (on[v]) swap(copy[0][0], copy[0][1]), swap(copy[1][0], copy[1][1]);
ans[0][0] = copy[0][0], ans[0][1] = copy[1][1] < 0 ? -1 : copy[1][1] + 1;
ans[1][0] = copy[0][1], ans[1][1] = copy[1][0] < 0 ? -1 : copy[1][0] + 1;
return ans;
}
int main() {
int n; cin >> n;
adj.resize(n), on.resize(n);
for (int i = 0; i < n - 1; ++i) {
int a, b; cin >> a >> b; --a, --b;
adj[a].push_back(b), adj[b].push_back(a);
}
for (int i = 0; i < n; ++i) cin >> on[i];
vi res = dfs(0, 0)[0];
int ans = res[0] < 0 || res[1] < 0 ? max(res[0], res[1]) : min(res[0], res[1]);
if (ans < 0) while (ans < 0);
cout << (ans < 0 ? "impossible" : to_string(ans)) << '\n';
}
