#include <bits/stdc++.h>
#define f first
#define s second
using namespace std;
const int N = 1e5 + 69;
const int inf = 1e9 + 69;
int n, dp[N][2][2], a[N];
vector<int> g[N];
void dfs(int v, int p) {
vector<int> ch;
for (auto u : g[v]) {
if (u == p)
continue;
dfs(u, v);
ch.push_back(u);
}
if (ch.empty()) {
dp[v][a[v]][0] = 0;
dp[v][a[v] ^ 1][1] = 1;
dp[v][a[v]][1] = dp[v][a[v] ^ 1][0] = inf;
}
else {
int uk1 = 0, uk2 = 0;
int mn1 = inf, mn2 = inf, c = 0;
for (auto u : ch) {
uk1 += min(dp[u][0][0], dp[u][0][1]);
if (dp[u][0][0] <= dp[u][0][1])
mn1 = min(mn1, dp[u][0][1] - dp[u][0][0]);
if (dp[u][0][1] <= dp[u][0][0]) {
mn2 = min(mn2, dp[u][0][0] - dp[u][0][1]);
c += 1;
}
}
if (c % 2 == 0) {
dp[v][a[v]][0] = uk1;
dp[v][a[v] ^ 1][0] = uk1 + min(mn1, mn2);
}
else {
dp[v][a[v] ^ 1][0] = uk1;
dp[v][a[v]][0] = uk1 + min(mn1, mn2);
}
mn1 = mn2 = inf;
c = 0;
for (auto u : ch) {
if (a[u] == 1) {
uk2 += min(dp[u][a[u]][0], dp[u][a[u]][1]);
if (dp[u][a[u]][0] <= dp[u][a[u]][1])
mn1 = min(mn1, dp[u][a[u]][1] - dp[u][a[u]][0]);
if (dp[u][a[u]][1] <= dp[u][a[u]][0]) {
mn2 = min(mn2, dp[u][a[u]][0] - dp[u][a[u]][1]);
c += 1;
}
}
else {
uk2 += min(dp[u][a[u] ^ 1][0], dp[u][a[u] ^ 1][1]);
if (dp[u][a[u] ^ 1][0] <= dp[u][a[u] ^ 1][1])
mn1 = min(mn1, dp[u][a[u] ^ 1][1] - dp[u][a[u] ^ 1][0]);
if (dp[u][a[u] ^ 1][1] <= dp[u][a[u] ^ 1][0]) {
mn2 = min(mn2, dp[u][a[u] ^ 1][0] - dp[u][a[u] ^ 1][1]);
c += 1;
}
}
}
if (c % 2 == 0) {
dp[v][a[v]][1] = uk2 + min(mn1, mn2) + 1;
dp[v][a[v] ^ 1][1] = uk2 + 1;
}
else {
dp[v][a[v]][1] = uk2 + 1;
dp[v][a[v] ^ 1][1] = uk2 + min(mn1, mn2) + 1;
}
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int tt = 1;
//cin >> tt;
while (tt--) {
cin >> n;
for (int i = 1;i <= n - 1;i++) {
int a, b;
cin >> a >> b;
g[a].push_back(b);
g[b].push_back(a);
}
for (int i = 1;i <= n;i++)
cin >> a[i];
dfs(1, 0);
int ans = min(dp[1][0][0], dp[1][0][1]);
if (ans >= inf)
cout << "impossible\n";
else
cout << ans << '\n';
}
return 0;
}
