// Starcraft 2 enjoyer //
#include <bits/stdc++.h>
// #pragma GCC target("avx2")
// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
using namespace std;
#define LSOne(X) ((X) & -(X))
#define int long long
const int N = 1e5 + 5;
const int M = 15e5;
const int B = 18;
const long long K = 2;
const int LG = 60;
const long long INF = 1e18 + 5;
const int C = 26;
const int MOD = 1e9 + 7;
int n, dp[N][2][2], s[N];
vector<int> adj[N];
void upd(int &a, int b)
{
if (a == -1)
a = b;
else
a = min(a, b);
}
void dfs(int c, int p)
{
int cur[2][2][2];
memset(cur, -1, sizeof(cur));
cur[0][0][0] = 0;
cur[0][1][0] = 0;
for (int i : adj[c])
{
if (i == p)
continue;
dfs(i, c);
if (dp[i][0][0] != -1)
{
if (cur[0][0][0] != -1)
upd(cur[1][0][0], cur[0][0][0] + dp[i][0][0]);
if (cur[0][0][1] != -1)
upd(cur[1][0][1], cur[0][0][1] + dp[i][0][0]);
}
if (dp[i][0][1] != -1)
{
if (cur[0][0][1] != -1)
upd(cur[1][0][0], cur[0][0][1] + dp[i][0][1]);
if (cur[0][0][0] != -1)
upd(cur[1][0][1], cur[0][0][0] + dp[i][0][1]);
}
if (dp[i][1][0] != -1)
{
if (cur[0][1][0] != -1)
upd(cur[1][1][0], cur[0][1][0] + dp[i][1][0]);
if (cur[0][1][1] != -1)
upd(cur[1][1][1], cur[0][1][1] + dp[i][1][0]);
}
if (dp[i][1][1] != -1)
{
if (cur[0][1][1] != -1)
upd(cur[1][1][0], cur[0][1][1] + dp[i][1][1]);
if (cur[0][1][0] != -1)
upd(cur[1][1][1], cur[0][1][0] + dp[i][1][1]);
}
// cout << c << " " << i << "\n";
for (int x = 0; x < 2; x++)
{
for (int y = 0; y < 2; y++)
{
cur[0][x][y] = cur[1][x][y];
cur[1][x][y] = -1;
// cout << cur[0][x][y] << " ";
}
}
// cout << "\n";
}
int i = s[c];
if (i)
{
dp[c][1][0] = cur[0][1][0];
dp[c][1][1] = cur[0][0][1];
dp[c][0][0] = cur[0][1][1];
dp[c][0][1] = cur[0][0][0];
}
else
{
dp[c][1][0] = cur[0][1][1];
dp[c][0][1] = cur[0][0][1];
dp[c][0][0] = cur[0][1][0];
dp[c][1][1] = cur[0][0][0];
}
if(dp[c][0][1] != -1) dp[c][0][1]++;
if(dp[c][1][1] != -1) dp[c][1][1]++;
if (c != p && adj[c].size() == 1)
{
dp[c][i][0] = 0;
dp[c][!i][1] = 1;
}
// cout << dp[c][0][0] << " " << dp[c][0][1] << " " << dp[c][1][0] << " " << dp[c][1][1] << " " << c << "\n";
}
inline void solve()
{
cin >> n;
for (int x = 0; x < n - 1; x++)
{
int a, b;
cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for (int x = 1; x <= n; x++)
{
cin >> s[x];
s[x] = !s[x];
}
memset(dp, -1, sizeof(dp));
dfs(1, 1);
int i = min(dp[1][1][0], dp[1][1][1]);
if (i == -1)
{
if(max(dp[1][1][0], dp[1][1][1]) == -1)
{
cout << "impossible\n";
}
else
{
cout << max(dp[1][1][0], dp[1][1][1]) << '\n';
}
}
else
cout << i;
}
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin >> t;
for (int x = 1; x <= t; x++)
{
solve();
}
return 0;
}
