#include <bits/stdc++.h>
#define fastio ios_base :: sync_with_stdio(0), cin.tie(0);
using namespace std;
using ll = long long;
const int mxN = 1e6 + 5, mod = 1e9 + 7;
int n, a[mxN], dp[mxN][2][2];
vector <int> adj[mxN];
void dfs(int v, int p = -1) {
int d[2][2], nd[2][2];
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j)
d[i][j] = nd[i][j] = n + 1;
}
bool ch = 0;
for (auto to : adj[v]) {
if (to != p) {
dfs(to, v);
if (!ch) {
d[0][0] = dp[to][0][0];
d[0][1] = dp[to][0][1];
d[1][0] = dp[to][1][0];
d[1][1] = dp[to][1][1];
ch = 1;
}
else {
nd[0][0] = min(d[0][0] + dp[to][0][0], d[0][1] + dp[to][0][1]);
nd[0][1] = min(d[0][0] + dp[to][0][1], d[0][1] + dp[to][0][0]);
nd[1][0] = min(d[1][0] + dp[to][1][0], d[1][1] + dp[to][1][1]);
nd[1][1] = min(d[1][0] + dp[to][1][1], d[1][1] + dp[to][1][0]);
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j)
d[i][j] = nd[i][j], nd[i][j] = n + 1;
}
}
}
}
if (!ch) {
if (!a[v]) {
dp[v][0][0] = 0;
dp[v][0][1] = n + 1;
dp[v][1][1] = 1;
dp[v][1][0] = n + 1;
}
else {
dp[v][0][0] = n + 1;
dp[v][0][1] = 1;
dp[v][1][0] = 0;
dp[v][1][1] = n + 1;
}
}
else {
if (!a[v]) {
dp[v][0][0] = d[1][0];
dp[v][0][1] = d[0][1] + 1;
dp[v][1][0] = d[1][1];
dp[v][1][1] = d[0][0] + 1;
} else {
dp[v][0][0] = d[1][1];
dp[v][0][1] = d[0][0] + 1;
dp[v][1][0] = d[1][0];
dp[v][1][1] = d[0][1] + 1;
}
}
}
int main() {
fastio;
cin >> n;
for (int i = 1, u, v; i < n; ++i) {
cin >> u >> v;
adj[u].push_back(v); adj[v].push_back(u);
}
for (int i = 1; i <= n; ++i) cin >> a[i], a[i] ^= 1;
dfs(1);
if (min(dp[1][1][0], dp[1][1][1]) > n)
cout << "impossible";
else cout << min(dp[1][1][0], dp[1][1][1]);
return 0;
}
