#include <bits/stdc++.h>
using namespace std;
#ifdef LOCAL
#include "debug.h"
#else
#define debug(...) 42
#endif
const int N = 1e5 + 5, inf = 1e9;
int n;
int dp[N][2][2], f[2][2], a[N];
vector<int> g[N];
void dfs(int u, int p) {
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j) {
f[i][j] = dp[u][i][j] = inf;
}
}
dp[u][a[u]][0] = 0;
dp[u][a[u] ^ 1][1] = 1;
for (int v : g[u]) {
if (v ^ p) {
dfs(v, u);
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j) {
f[j ^ i][0] = min(f[j ^ i][0], dp[u][j][0] + dp[v][0][i]);
f[j ^ i][1] = min(f[j ^ i][1], dp[u][j][1] + dp[v][1][i]);
}
}
for (int i = 0; i < 2; ++i) {
for (int j = 0; j < 2; ++j) {
dp[u][i][j] = f[i][j];
f[i][j] = inf;
}
}
}
}
}
int main() {
ios::sync_with_stdio(false); cin.tie(nullptr);
cin >> n;
for (int i = 1; i < n; ++i) {
int u, v; cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
for (int i = 1; i <= n; ++i) {
cin >> a[i];
}
dfs(1, 1);
int res = min(dp[1][0][0], dp[1][0][1]);
if (res == inf) {
cout << "impossible";
} else {
cout << res;
}
return 0;
}
