#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int N = 100100;
const int INF = 1e9;
int n;
vector<int> g[N];
int a[N];
int dp[N][2][2];
void dfs(int s, int p) {
for (int to : g[s]) if (to != p) {
dfs(to, s);
}
for (int x = 0; x <= 1; x++) {
int f[2]{0, INF};
for (int to : g[s]) if (to != p) {
bool t = (x ^ a[to]);
int g[2]{INF, INF};
for (int xc = 0; xc <= 1; xc++) {
for (int yc = 0; yc <= 1; yc++) {
if ((xc ^ yc) != (t)) continue;
for (int pr = 0; pr <= 1; pr++) {
g[(pr ^ xc)] = min(g[(pr ^ xc)], f[pr] + dp[to][xc][yc]);
}
}
}
f[0] = g[0];
f[1] = g[1];
}
dp[s][x][0] = f[0] + x;
dp[s][x][1] = f[1] + x;
}
}
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
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 = INF;
for (int x = 0; x <= 1; x++) for (int y = 0; y <= 1; y++)
if ((x ^ y) == a[1])
res = min(res, dp[1][x][y]);
if (res == INF) {
cout << "impossible\n";
return 0;
} else {
cout << res << '\n';
return 0;
}
}
