#include <bits/stdc++.h>
#define ll long long
#define pii pair<int, int>
#define pll pair<ll, ll>
#define F first
#define S second
using namespace std;
// dp[op][cur]
// op = 1: all subtree cur == 0
// op = 0: all subtree cur == 1
ll N, p[100005], a[100005], dp[100005][2][2];
vector<int> E[100005];
void dfs(int u)
{
// dp[u][0][0] = dp[u][0][1] = dp[u][1][0] = dp[u][1][1] = N + 1;
ll dcur = a[u], ncur = a[u], d = N + 1, n = N + 1, dcost = 0, ncost = 0;
for (int v : E[u])
if (p[u] != v)
{
//cerr << u << " -> " << v << '\n';
p[v] = u;
dfs(v);
dcur ^= (dp[v][0][1] < dp[v][1][1] ? 0 : 1);
dcost += min(dp[v][0][1], dp[v][1][1]);
d = min(d, abs(dp[v][0][1] - dp[v][1][1]));
ncur ^= (dp[v][0][0] < dp[v][1][0] ? 0 : 1);
ncost += min(dp[v][0][0], dp[v][1][0]);
n = min(n, abs(dp[v][0][0] - dp[v][1][0]));
}
dp[u][1][dcur ^ 1] = dcost + 1;
dp[u][1][dcur] = dcost + d + 1;
dp[u][0][ncur] = ncost;
dp[u][0][ncur ^ 1] = ncost + n;
//cerr << u << ": " << dp[u][0][0] << ' ' << dp[u][0][1] << ' ' << dp[u][1][0] << ' ' << dp[u][1][1] << '\n';
}
signed main()
{
ios::sync_with_stdio(0);
cin.tie(0), cout.tie(0);
cin >> N;
for (int i = 1; i < N; i++)
{
int u, v;
cin >> u >> v;
E[u].emplace_back(v);
E[v].emplace_back(u);
}
for (int i = 1; i <= N; i++)
cin >> a[i];
dfs(1);
int ans = min(dp[1][0][0], dp[1][1][0]);
if (ans > N)
cout << "impossible\n";
else
cout << ans << '\n';
}
