#include <bits/stdc++.h>
using namespace std;
#define int long long
const int MAXN=1e5+10;
const int INF=1e9;
int n,a[MAXN],dp[2][2][MAXN];
vector <int> g[MAXN];
///everybody has to be 0
void dfs (int node, int prevn){
bool ok=0;
for (auto x:g[node]){
if (x==prevn) continue;
dfs (x,node);
ok=1;
}
///node is leaf
if (ok==0){
dp[a[node]][0][node]=0;
dp[1-a[node]][1][node]=1;
return;
}
///if node is not using than all of his sons must be 0
int s=0,nr=0,mind=INF;
for (auto x:g[node]){
if (x==prevn) continue;
///is x used or not
if (dp[0][1][x]<dp[0][0][x]){
s+=dp[0][1][x];
nr++;
}
else{
s+=dp[0][0][x];
}
if (dp[0][1][x]!=INF and dp[0][0][x]!=INF)
mind=min (mind,abs (dp[0][1][x]-dp[0][0][x]));
}
dp[(nr+a[node])%2][0][node]=s;
dp[(nr+a[node]+1)%2][0][node]=min (INF,s+mind);
///if node is used than all of his sons must be 1
s=0,nr=0,mind=INF;
for (auto x:g[node]){
if (x==prevn) continue;
if (dp[1][1][x]<dp[1][0][x]){
s+=dp[1][1][x];
nr++;
}
else{
s+=dp[1][0][x];
}
if (dp[1][1][x]!=INF and dp[1][0][x]!=INF)
mind=min (mind,abs (dp[1][1][x]-dp[1][0][x]));
}
///I am pressing 1 so add 1
dp[(nr+a[node]+1)%2][1][node]=s+1;
dp[(nr+a[node])%2][1][node]=min (INF,s+mind+1);
}
signed main()
{
ios_base::sync_with_stdio (false);
cin.tie (nullptr);
cin >>n;
for (int i=1;i<n;++i){
int x,y;
cin >>x>>y;
g[x].push_back (y);
g[y].push_back (x);
}
for (int i=1;i<=n;++i){
cin >>a[i];
dp[0][0][i]=dp[0][1][i]=dp[1][0][i]=dp[1][1][i]=INF;
}
dfs (1,1);
int rez=min (dp[0][0][1],dp[0][1][1]);
if (rez>=INF){
cout <<"impossible";
}
else{
cout <<rez;
}
return 0;
}
