#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace std;
int t = 0;
int const f =1e5 + 10;
long long mod = 1e9 + 7;//998244353;
long long dp[f];
long long dp1[f];
long long dp2[f];
long long dp3[f];
// long long par1[f];
// long long par2[f];
long long a[f];
// long long adj[f];
// long long mmn =0;
// long long C[4];
// pair<long long, long long> dd[f];
// long long prefix[f];
vector<int>g[f];
vector<pair<long long,long long>>gr;
struct node {
long long val = 0;
long long lazy = 0;
long long prefix =0;
long long suffix =0;
long long all =0;
};
// struct trpel {
// long long v, u, bank = 0;
// };
node segtree[f *4];
bool B[f];
void redany() {
ifstream fin;
ofstream fout;
fin.open("std.in");
fout.open("std.out");
}
long long powmod(long long a, long long p, long long modd) {
long long ans = 1;
while (p > 0) {
if (p % 2 == 1) {
ans *= a;
ans %= modd;
p--;
}
if (p == 0)break;
a *= a;
a %= modd;
p /= 2;
}
return ans;
}
void dolazy(int v){
long long k = segtree[v].lazy;
segtree[v].lazy = 0;
segtree[2*v].lazy+=k;
segtree[2*v+1].lazy +=k;
segtree[2*v].val+=k;
segtree[2*v+1].val+=k;
}
void upd(int v, int tl, int tr, int l ,int r, int val){
if(tr<l||tl>r)return;
if(tr<=r&&tl>=l){
segtree[v].val+=val;
segtree[v].lazy+=val;
return;
}
dolazy(v);
int mid = (tl+tr)/2;
upd(2*v,tl,mid,l,r,val);
upd(2*v+1,mid+1,tr,l,r,val);
segtree[v].val = max(segtree[2*v].val,segtree[2*v+1].val);
}
long long q(int v, int tl , int tr,int l , int x){
if(tr<l||segtree[v].val<x)return 1e18;
if(tr==tl)return tr;
int mid = (tr+tl)/2;
dolazy(v);
if(tl>=l&&segtree[v].val>=x){
if(segtree[v*2].val>=x){
return q(2*v,tl,mid,l,x);
}
return q(2*v+1,mid+1,tr,l,x);
}
return min(q(2*v,tl,mid,l,x),q(2*v+1,mid+1,tr,l,x));
}
void dfs(int v, int parv){
for(int u:g[v]){
if(u==parv)continue;
dfs(u,v);
}
if(g[v].size()==1&&parv!=0){
if(B[v]){
dp2[v] = 1e18;
dp1[v] = 1e18;
dp[v] = 1;
dp3[v] = 0;
}
else{
dp[v] = dp3[v] = 1e18;
dp1[v] = 1;
dp2[v] =0;
}
//cout<<v<<" : "<<dp[v]<<" "<<dp1[v]<<" "<<dp2[v]<<" "<<dp3[v]<<'\n';
return;
}
long long a =0, b=0;
// dp me yes off
//dp1 me yes on
// dp2 me no off
// dp3 me no on
bool c=0;
dp[v] = 1;
for(int u:g[v]){
if(u==parv)continue;
if(min(dp1[u],dp3[u])==1e18){
c=1;
//cout<<v<<'\n';
dp[v] = 1e18;break;
}
dp[v]+=min(dp1[u],dp3[u]);
if(dp1[u]<dp3[u]){
a++;
}
else{
b++;
}
}
if((a+1+B[v])%2==1&&!c){
a = b =1e18;
for(int u:g[v]){
if(u==parv)continue;
if(dp1[u]<dp3[u]){
if(dp3[u]!=1e18){
a = min(a,dp3[u]-dp1[u]);
}
}
else{
if(dp1[u]!=1e18){
a = min(a,dp1[u]-dp3[u]);
}
}
}
if(a==1e18){
dp[v] = 1e18;
}
else{
dp[v]+=a;
}
}
dp1[v] =1;
a =0;
c=0;
for(int u:g[v]){
if(u==parv)continue;
if(min(dp1[u],dp3[u])==1e18){
c=1;
dp1[v] = 1e18;break;
}
dp1[v]+=min(dp1[u],dp3[u]);
if(dp1[u]<dp3[u]){
a++;
}
else{
b++;
}
}
if((a+1+B[v])%2==0&&!c){
a = b =1e18;
for(int u:g[v]){
if(u==parv)continue;
if(dp1[u]<dp3[u]){
if(dp3[u]!=1e18){
a = min(a,dp3[u]-dp1[u]);
}
}
else{
if(dp1[u]!=1e18){
a = min(a,dp1[u]-dp3[u]);
}
}
}
if(a==1e18){
dp1[v] = 1e18;
}
else{
dp1[v]+=a;
}
}
dp2[v] =0;
a=0;
c=0;
for(int u:g[v]){
if(u==parv)continue;
if(min(dp[u],dp2[u])==1e18){
c=1;
dp2[v] = 1e18;break;
}
dp2[v]+=min(dp[u],dp2[u]);
if(dp[u]<dp2[u]){
a++;
}
else{
b++;
}
}
if((a+B[v])%2==1&&!c){
a = b =1e18;
for(int u:g[v]){
if(u==parv)continue;
if(dp[u]<dp2[u]){
if(dp2[u]!=1e18){
a = min(a,dp2[u]-dp[u]);
}
}
else{
if(dp[u]!=1e18){
a = min(a,dp[u]-dp2[u]);
}
}
}
if(a==1e18){
dp2[v] = 1e18;
}
else{
dp2[v]+=a;
}
}
dp3[v] = 0;
a=0;
c=0;
for(int u:g[v]){
if(u==parv)continue;
if(min(dp[u],dp2[u])==1e18){
c=1;
dp3[v] = 1e18;break;
}
dp3[v]+=min(dp[u],dp2[u]);
if(dp[u]<dp2[u]){
a++;
}
else{
b++;
}
}
if((a+B[v])%2==0&&!c){
a = b =1e18;
for(int u:g[v]){
if(u==parv)continue;
if(dp[u]<dp2[u]){
if(dp2[u]!=1e18){
a = min(a,dp2[u]-dp[u]);
}
}
else{
if(dp[u]!=1e18){
a = min(a,dp[u]-dp2[u]);
}
}
}
if(a==1e18){
dp3[v] = 1e18;
}
else{
dp3[v]+=a;
}
}
//cout<<v<<" : "<<dp[v]<<" "<<dp1[v]<<" "<<dp2[v]<<" "<<dp3[v]<<'\n';
}
long long tw = powmod(2, mod - 2, mod);
int main() {
ios::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
t = 1;
//cin >> t;
for (int hhh = 1;hhh <= t;hhh++) {
long long n, mx = 0, d = 0, m = 0, k = 0, k2 = 1, r = 0, rr = 0, l = 0, ll = 0, lll = 0, rrr = 0, inf = 1e18;
string s1;
string s3;
char s2;
long double dec = 0;
bool c = 1;
bool allrows =0;
bool allculms =0;
pair<long long,long long> aa;
priority_queue<pair<long long, long long>> qq;
multiset<long long> mul;
multiset<long long> mul2;
set<long long>st;
queue<long long> pp;
map<long long, long long> conv;
map<long long,long long> convback;
vector<long long> vec;
cin >>n;
for(int i =0;i<n-1;i++){
cin>>r>>l;
g[r].push_back(l);
g[l].push_back(r);
}
for(int i =1;i<=n;i++){
cin>>B[i];
}
dfs(1,0);
if(min(dp[1],dp2[1])==1e18)
cout<<"impossible";
else cout<<min(dp[1],dp2[1]);
// m = 1e12;
// cout<<m%mod;
}
}
//kir
