#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 1e5 + 7;
int a[N], dp[N][2];
vector<int> g[N];
void dfs(int u, int p = 0) {
vector<int> kids;
for (auto v : g[u]) {
if (v != p) {
dfs(v, u);
kids.push_back(v);
}
}
if (kids.size() == 0) {
dp[u][0] = 0;
dp[u][1] = 1;
return;
}
for (int val = 0; val < 2; ++val) {
vector<int> mn(2, 1e9);
if (val == 1) {
mn[1] = 1;
} else {
mn[0] = 0;
}
for (auto v : kids) {
vector<int> nwmn(2, 1e9);
for (int i = 0; i < 2; ++i) {
for (int s = 0; s < 2; ++s) {
if (s ^ val != a[v]) continue;
nwmn[i ^ s] = min(nwmn[i ^ s], mn[i] + dp[v][s]);
}
}
swap(nwmn, mn);
}
dp[u][0] = min(dp[u][0], mn[0]);
dp[u][1] = min(dp[u][1], mn[1]);
}
}
int main() {
ios::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
for (int i = 1; i < n; ++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];
}
for (int i = 1; i <= n; ++i) dp[i][0] = dp[i][1] = 1e9;
dfs(1);
if (dp[1][a[1]] == 1e9) {
cout << "impossible\n";
return 0;
}
cout << dp[1][a[1]] << "\n";
return 0;
}
