#include <bits/stdc++.h>
using namespace std;
#define fast ios::sync_with_stdio(0);cin.tie(0);
#define s second
#define f first
typedef long long ll;
const ll MOD = 1e9+7;
const ll P = 31;
const ll LOGN = 20;
const ll MAXN = 1e5 + 100;
vector<vector<int>> graph;
int active[MAXN], dp[MAXN][2][2], temp_dp[2];
void act(int node, int parent) {
temp_dp[0] = 0;
temp_dp[1] = 1e8;
for (auto u : graph[node]) {
if (u == parent)
continue;
int td0 = temp_dp[0], td1 = temp_dp[1];
temp_dp[0] = min(td1 + dp[u][1][1], td0 + dp[u][0][1]);
temp_dp[1] = min(td1 + dp[u][0][1], td0 + dp[u][1][1]);
}
dp[node][1][0] = temp_dp[0] + 1;
dp[node][1][1] = temp_dp[1] + 1;
temp_dp[0] = 0;
temp_dp[1] = 1e8;
for (auto u : graph[node]) {
if (u == parent)
continue;
int td0 = temp_dp[0], td1 = temp_dp[1];
temp_dp[0] = min(td1 + dp[u][1][0], td0 + dp[u][0][0]);
temp_dp[1] = min(td1 + dp[u][0][0], td0 + dp[u][1][0]);
}
dp[node][0][1] = temp_dp[0];
dp[node][0][0] = temp_dp[1];
}
void deact(int node, int parent) {
temp_dp[0] = 0;
temp_dp[1] = 1e8;
for (auto u : graph[node]) {
if (u == parent)
continue;
int td0 = temp_dp[0], td1 = temp_dp[1];
temp_dp[0] = min(td1 + dp[u][1][1], td0 + dp[u][0][1]);
temp_dp[1] = min(td1 + dp[u][0][1], td0 + dp[u][1][1]);
}
dp[node][1][1] = temp_dp[0] + 1;
dp[node][1][0] = temp_dp[1] + 1;
temp_dp[0] = 0;
temp_dp[1] = 1e8;
for (auto u : graph[node]) {
if (u == parent)
continue;
int td0 = temp_dp[0], td1 = temp_dp[1];
temp_dp[0] = min(td1 + dp[u][1][0], td0 + dp[u][0][0]);
temp_dp[1] = min(td1 + dp[u][0][0], td0 + dp[u][1][0]);
}
dp[node][0][0] = temp_dp[0];
dp[node][0][1] = temp_dp[1];
}
void dfs(int node, int parent) {
if (graph[node].size() == 1 && node != 1) {
if (active[node]) {
dp[node][1][0] = 1;
dp[node][0][1] = 0;
dp[node][1][1] = 1e8;
dp[node][0][0] = 1e8;
} else {
dp[node][1][1] = 1;
dp[node][0][0] = 0;
dp[node][1][0] = 1e8;
dp[node][0][1] = 1e8;
}
return ;
}
for (auto u : graph[node]) {
if (u == parent)
continue;
dfs(u, node);
}
if (active[node])
act(node, parent);
else
deact(node, parent);
}
int main() {
fast
int N, a, b;
cin >> N;
graph = vector<vector<int>>(N+1, vector<int>());
for (int i = 1; i <= N; i++) {
for (int j = 0; j < 2; j++)
for (int q = 0; q < 2; q++)
dp[i][j][q] = 1e8;
}
for (int i = 1; i < N; i++) {
cin >> a >> b;
graph[a].push_back(b);
graph[b].push_back(a);
}
for (int i = 1; i <= N; i++)
cin >> active[i];
dfs(1, 1);
if (min(dp[1][0][0], dp[1][1][0]) >= 1e8)
cout << "impossible\n";
else
cout << min(dp[1][0][0], dp[1][1][0]) << "\n";
}
