#include <cstdio>
#include <stdio.h>
#include <stdbool.h>
#include <iostream>
#include <map>
#include <vector>
#include <climits>
#include <stack>
#include <string>
#include <queue>
#include <algorithm>
#include <set>
#include <unordered_set>
#include <unordered_map>
#include <cmath>
#include <cctype>
#include <bitset>
#include <iomanip>
#include <cstring>
#include <numeric>
#include <cassert>
#include <random>
using namespace std;
#define int long long
#define pii pair<int, int>
#define mp make_pair
#define pb push_back
#define fi first
#define se second
vector<int> vect;
vector<vector<int> > graph;
vector<vector<vector<int> > > dp;
void dfs(int node, int p){
if (graph[node].size()==1&&p){
dp[node][vect[node]][0]=0;
dp[node][!vect[node]][1]=1;
return;
}
int sum0=0, sum1=0, d0=LLONG_MAX/2, d1=LLONG_MAX/2, c0=0, c1=0;
for (auto num:graph[node])if (num!=p){
dfs(num, node);
sum0+=min(dp[num][0][0], dp[num][0][1]);
sum1+=min(dp[num][1][0], dp[num][1][1]);
if (dp[num][0][1]<=dp[num][0][0])++c0;
if (dp[num][1][1]<=dp[num][1][0])++c1;
d0=min(d0, max(dp[num][0][0], dp[num][0][1])-min(dp[num][0][0], dp[num][0][1]));
d1=min(d1, max(dp[num][1][0], dp[num][1][1])-min(dp[num][1][0], dp[num][1][1]));
}
dp[node][vect[node]^(c0&1)][0]=sum0;
dp[node][vect[node]^(c0&1)^1][0]=sum0+d0;
dp[node][vect[node]^(c1&1)^1][1]=1+sum1;
dp[node][vect[node]^(c1&1)][1]=1+sum1+d1;
for (int i=0; i<2; ++i)for (int j=0; j<2; ++j)dp[node][i][j]=min(dp[node][i][j], LLONG_MAX/2);
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int n, a, b;
cin>>n;
vect.resize(n+1);
graph.resize(n+1);
dp.resize(n+1, vector<vector<int> >(2, vector<int>(2, LLONG_MAX/2)));
for (int i=1; i<n; ++i){
cin>>a>>b;
graph[a].pb(b);
graph[b].pb(a);
}
for (int i=1; i<=n; ++i)cin>>vect[i];
dfs(1, 0);
if (min(dp[1][0][0], dp[1][0][1])==LLONG_MAX/2)cout<<"impossible";
else cout<<min(dp[1][0][0], dp[1][0][1]);
}
