#include <bits/stdc++.h>
using namespace std;
const int N = (int)1e5 + 7;
const int INF = (int)1e9 + 7;
int n;
int v[N];
int cost[N][2][2];
int nw[2][2];
vector<int> g[N];
void minup(int& a, int b) {
a = min(a, b);
}
void dfs(int a, int p = -1) {
vector<int> kids;
for (auto& b : g[a]) {
if (b == p) {
continue;
}
kids.push_back(b);
dfs(b, a);
}
g[a] = kids;
cost[a][0][0] = cost[a][0][1] = cost[a][1][0] = cost[a][1][1] = INF;
cost[a][0][v[a]] = 0;
cost[a][1][v[a] ^ 1] = 1;
for (auto& b : g[a]) {
nw[0][0] = nw[0][1] = nw[1][0] = nw[1][1] = INF;
for (int apply = 0; apply <= 1; apply++) {
for (int cur = 0; cur <= 1; cur++) {
for (int apply_b = 0; apply_b <= 1; apply_b++) {
minup(nw[apply][cur ^ apply_b], cost[a][apply][cur] + cost[b][apply_b][apply]);
}
}
}
for (int i = 0; i <= 1; i++) {
for (int j = 0; j <= 1; j++) {
cost[a][i][j] = nw[i][j];
}
}
}
}
int main() {
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
///freopen("___input___.txt","r",stdin);
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 >> v[i];
}
dfs(1);
int sol = INF;
for (int x = 0; x <= 1; x++) {
sol = min(sol, cost[1][x][0]);
}
if (sol == INF) {
cout << "impossible\n";
return 0;
}
cout << sol << "\n";
return 0;
}
