#include <bits/stdc++.h>
typedef long long ll;
using namespace std;
const int INF = 1e9;
vector<int> graph[100001];
bool white[100001];
int dp[100001][2][2];
void dfs(int node = 1, int parent = 0) {
int case_1 = 0, case_2 = 0;
int switch_1 = INF, switch_2 = INF;
bool parity_1 = !white[node], parity_2 = white[node];
for (int i : graph[node]) if (i != parent) {
dfs(i, node);
// Case 1: We switch node
if (dp[i][1][0] < dp[i][1][1]) {
case_1 += dp[i][1][0];
switch_1 = min(switch_1, dp[i][1][1] - dp[i][1][0]);
} else {
case_1 += dp[i][1][1];
switch_1 = min(switch_1, dp[i][1][0] - dp[i][1][1]);
parity_1 ^= true;
}
// Case 2: We don't switch node
if (dp[i][0][0] < dp[i][0][1]) {
case_2 += dp[i][0][0];
switch_2 = min(switch_2, dp[i][0][1] - dp[i][0][0]);
} else {
case_2 += dp[i][0][1];
switch_2 = min(switch_2, dp[i][0][0] - dp[i][0][1]);
parity_2 ^= true;
}
}
dp[node][parity_1][1] = case_1 + 1;
dp[node][!parity_1][1] = dp[node][parity_1][1] + switch_1;
dp[node][parity_2][0] = case_2;
dp[node][!parity_2][0] = dp[node][parity_2][0] + switch_2;
}
int main() {
cin.tie(0)->sync_with_stdio(0);
int n;
cin >> n;
for (int i = 1; i < n; i++) {
int u, v;
cin >> u >> v;
graph[u].push_back(v);
graph[v].push_back(u);
}
for (int i = 1; i <= n; i++) cin >> white[i];
dfs();
int ans = min(dp[1][0][0], dp[1][0][1]);
if (ans > n) cout << "impossible";
else cout << ans;
return 0;
}
