#include <iostream>
#include <vector>
#define pb push_back
using namespace std;
const int N = 1e5 + 1;
const int inf = 1e9;
bool turned[N];
vector <int> g[N];
int dp[N][2][2], tmp[2][2];
void dfs(int v, int p) {
for (int to : g[v]) {
if (to != p) {
dfs(to, v);
}
}
dp[v][turned[v]][0] = 0;
dp[v][turned[v]][1] = inf;
dp[v][1 ^ turned[v]][1] = 1;
dp[v][1 ^ turned[v]][0] = inf;
for (int to : g[v]) {
if (to != p) {
tmp[0][0] = min(tmp[0][0], dp[v][0][0] + dp[to][0][0]);
tmp[0][0] = min(tmp[0][0], dp[v][1][0] + dp[to][0][1]);
tmp[0][1] = min(tmp[0][1], dp[v][0][1] + dp[to][1][0]);
tmp[0][1] = min(tmp[0][1], dp[v][1][1] + dp[to][1][1]);
tmp[1][0] = min(tmp[1][0], dp[v][0][0] + dp[to][0][1]);
tmp[1][0] = min(tmp[1][0], dp[v][1][0] + dp[to][0][0]);
tmp[1][1] = min(tmp[1][1], dp[v][0][1] + dp[to][1][1]);
tmp[1][1] = min(tmp[1][1], dp[v][1][1] + dp[to][1][0]);
for (int i = 0; i < 2; i++) {
for (int j = 0; j < 2; j++) {
dp[v][i][j] = tmp[i][j];
tmp[i][j] = inf;
}
}
}
}
}
int main(){
int n;
cin >> n;
for (int i = 1; i < n; i++) {
int u, v;
cin >> u >> v;
g[u].pb(v);
g[v].pb(u);
}
for (int i = 1; i <= n; i++) {
cin >> turned[i];
}
for (int i : {0, 1}) {
for (int j : {0, 1}) {
tmp[i][j] = inf;
}
}
dfs(1, 1);
int ans = min(dp[1][0][0], dp[1][0][1]);
if (ans == inf) cout << "impossible";
else cout << ans;
}
