#include<bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef long double ld;
#define sync ios::sync_with_stdio(false),cin.tie(0),cout.tie(0)
#define pb push_back
#define pii pair<int, int>
#define mp make_pair
#define fi first
#define se second
const int maxn = 1e5 + 2, inf = 1e7;
vector<int> adj[maxn];
int dp[maxn][2][2];
int sit[maxn];
int n;
void Dfs(int v, int p)
{
for(int u : adj[v])
{
if(u != p)
Dfs(u, v);
}
if(adj[v].size() == 1)
{
dp[v][sit[v]][0] = 0;
dp[v][1 - sit[v]][1] = 1;
return;
}
int sum[2] = {0, 0}, w[2] = {inf, inf}, cnt[2] = {0, 0};
for(int u : adj[v])
{
if(u == p)
continue;
sum[0] += min(dp[u][0][0], dp[u][0][1]);
sum[1] += min(dp[u][1][0], dp[u][1][1]);
w[0] = min(w[0], max(dp[u][0][0], dp[u][0][1]) - min(dp[u][0][0], dp[u][0][1]));
w[1] = min(w[1], max(dp[u][1][0], dp[u][1][1]) - min(dp[u][1][0], dp[u][1][1]));
cnt[0] += (dp[u][0][0] >= dp[u][0][1]);
cnt[1] += (dp[u][1][0] >= dp[u][1][1]);
}
dp[v][0][0] = sum[0];
dp[v][1][0] = sum[0];
dp[v][0][1] = sum[1] + 1;
dp[v][1][1] = sum[1] + 1;
if(sit[v] == 1)
{
if(cnt[0] % 2)
dp[v][1][0] += w[0];
else
dp[v][0][0] += w[0];
if(cnt[1] % 2)
dp[v][0][1] += w[1];
else
dp[v][1][1] += w[1];
}
else
{
if(cnt[0] % 2)
dp[v][0][0] += w[0];
else
dp[v][1][0] += w[0];
if(cnt[1] % 2)
dp[v][1][1] += w[1];
else
dp[v][0][1] += w[1];
}
/* cout << v << " : " << endl;
cout << dp[v][0][0] << " , " << dp[v][0][1] << " , " << dp[v][1][0] << " , " << dp[v][1][1] << endl;
cout << sum[0] << " + " << w[0] << " , " << sum[1] << " + " << w[1] << endl;*/
return;
}
int main()
{
sync;
cin >> n;
int r = 0;
for(int i = 0; i < n-1; i++)
{
int v, u;
cin >> v >> u;
v--, u--;
adj[v].pb(u);
adj[u].pb(v);
if(adj[v].size() > 1)
r = v;
if(adj[u].size() > 1)
r = u;
}
for(int i = 0; i < n; i++)
cin >> sit[i];
for(int i = 0; i < n; i++)
for(int j = 0; j < 2; j++)
for(int k = 0; k < 2; k++)
dp[i][j][k] = inf;
//memset(dp, 63, sizeof dp);
Dfs(r, -1);
int ans = min(dp[r][0][0], dp[r][0][1]);
if(ans > n)
cout << "impossible" << '\n';
else
cout << ans << '\n';
return 0;
}
