#include <iostream>
#include <vector>
#define MAX 100000
#define INF 1000000000
using namespace std;
///ifstream cin ("c.in");
///ofstream cout ("c.out");
int status[MAX + 10];
long long DP[MAX + 10][3][3];
vector <int> graph[MAX + 10];
void dfs(int node, int parent)
{
long long dp[3][3];
dp[0][0] = 0;
dp[0][1] = 0;
dp[1][0] = INF;
dp[1][1] = INF;
for (auto next : graph[node])
if (next != parent)
{
dfs(next, node);
long long aux[3][3];
aux[0][0] = dp[0][0];
aux[0][1] = dp[0][1];
aux[1][0] = dp[1][0];
aux[1][1] = dp[1][1];
dp[0][0] = min(aux[0][0] + DP[next][0][0], aux[1][0] + DP[next][1][0]);
dp[0][1] = min(aux[0][1] + DP[next][0][1], aux[1][1] + DP[next][1][1]);
dp[1][0] = min(aux[0][0] + DP[next][1][0], aux[1][0] + DP[next][0][0]);
dp[1][1] = min(aux[0][1] + DP[next][1][1], aux[1][1] + DP[next][0][1]);
}
if (status[node] == 1)
{
DP[node][0][0] = dp[1][0];
DP[node][0][1] = dp[0][0];
DP[node][1][0] = dp[0][1] + 1;
DP[node][1][1] = dp[1][1] + 1;
}
else
{
DP[node][0][0] = dp[0][0];
DP[node][0][1] = dp[1][0];
DP[node][1][0] = dp[1][1] + 1;
DP[node][1][1] = dp[0][1] + 1;
}
}
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
int n;
cin >> n;
for (int i = 1; i < n; i++)
{
int x, y;
cin >> x >> y;
graph[x].push_back(y);
graph[y].push_back(x);
}
for (int i = 1; i <= n; i++)
cin >> status[i];
dfs(1, 0);
long long ans = min(DP[1][0][0], DP[1][1][0]);
if (ans <= n)
cout << ans;
else
cout << "impossible";
return 0;
}
