#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define vb vector<bool>
#define vi vector<int>
#define vpii vector<pii>
#define vvb vector<vb>
#define vvi vector<vi>
#define vvvi vector<vvi>
#define vvpii vector<vpii>
#define sz(x) (int)(x).size()
int N;
vi L;
vvi adj;
vvvi dp;
void dfs(int v, int p)
{
for (int u : adj[v])
if (u != p)
dfs(u, v);
for (int schalten = 0; schalten < 2; ++schalten)
{
vvi res(1, vi(2, INT_MAX));
res[0][L[v] ^ schalten] = schalten;
int i = 0;
for (int u : adj[v])
{
if (u == p)
continue;
res.push_back(vi(2, INT_MAX));
++i;
if (dp[u][schalten][0] != INT_MAX)
{
if (res[i - 1][0] != INT_MAX)
res[i][0] = min(res[i][0], res[i - 1][0] + dp[u][schalten][0]);
if (res[i - 1][1] != INT_MAX)
res[i][1] = min(res[i][1], res[i - 1][1] + dp[u][schalten][0]);
}
if (dp[u][schalten][1] != INT_MAX)
{
if (res[i - 1][1] != INT_MAX)
res[i][0] = min(res[i][0], res[i - 1][1] + dp[u][schalten][1]);
if (res[i - 1][0] != INT_MAX)
res[i][1] = min(res[i][1], res[i - 1][0] + dp[u][schalten][1]);
}
}
dp[v][0][schalten] = res.back()[0];
dp[v][1][schalten] = res.back()[1];
}
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin >> N;
adj.resize(N);
for (int i = 0, a, b; i < N - 1; ++i)
{
cin >> a >> b;
--a, --b;
adj[a].push_back(b);
adj[b].push_back(a);
}
L.resize(N);
for (int i = 0; i < N; ++i)
cin >> L[i];
dp.assign(N, vvi(2, vi(2, INT_MAX))); // [node][res][wurde geschaltet?]
dfs(0, 0);
int ans = min(dp[0][0][0], dp[0][0][1]);
if (ans != INT_MAX)
cout << ans << "\n";
else
cout << "impossible\n";
return 0;
}
