#include <bits/stdc++.h>
using namespace std;
#define ar array
#define sz(v) static_cast<int>(v.size())
#define all(v) (v).begin(), (v).end()
typedef pair<int, int> pi;
typedef long long ll;
typedef vector<int> vi;
const int N = 1e5, INF = 1e9;
int n, a[N];
ar<int, 2> dp[N][2];
vi g[N];
ar<int, 2> operator+(const ar<int, 2> &u, const ar<int, 2> &v) {
return {min({INF, u[0] + v[0], u[1] + v[1]}),
min({INF, u[0] + v[1], u[1] + v[0]})};
}
void dfs(int p, int i) {
ar<int, 2> op0{0, INF}, op1{INF, 1};
for (int j : g[i])
if (p ^ j) {
dfs(i, j);
op0 = op0 + dp[j][0];
op1 = op1 + dp[j][1];
}
if (a[i]) {
swap(op0[0], op0[1]);
swap(op1[0], op1[1]);
}
dp[i][0][0] = op0[0];
dp[i][1][0] = op0[1];
dp[i][0][1] = op1[0];
dp[i][1][1] = op1[1];
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n;
for (int h = 0; h < n - 1; h++) {
int i, j;
cin >> i >> j, i--, j--;
g[i].push_back(j), g[j].push_back(i);
}
for (int i = 0; i < n; i++) cin >> a[i];
dfs(-1, 0);
int x = min(dp[0][0][0], dp[0][0][1]);
if (x == INF)
cout << "impossible\n";
else
cout << x << '\n';
}
