// Only GOD
// believe in yourself
// nemidam del be in darde donya!
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef long double ld;
typedef pair<int, int> pii;
#define F first
#define S second
#define pb push_back
#define all(x) x.begin(), x.end()
#define bit(x, y) ((x >> y)&1)
#define sz(x) (int)x.size()
#define kill(x) return cout << x << '\n', void()
#define KILL(x) return cout << x << '\n', 0
const int N = 1e5+10;
const int INF = 1e9;
int dp[N][4];
bool f[N];
vector<int> G[N];
void upd1(int v, int p){
int cnt = 0;
bool C = 0;
for(int u : G[v]){
if(u == p)
continue;
C |= (dp[u][0] >= INF && dp[u][2] >= INF);
if(dp[u][0] < dp[u][2]){
dp[v][0] += dp[u][0];
}
else{
dp[v][0] += dp[u][2];
cnt++;
}
}
if(C){
dp[v][0] = INF;
dp[v][1] = INF;
return;
}
dp[v][1] = dp[v][0];
if((f[v] && cnt%2==0) || (!f[v] && cnt % 2)){
int mn = INF;
for(int u : G[v]){
if(u==p)
continue;
if(dp[u][0] < INF && dp[u][2] < INF){
mn = min(mn, abs(dp[u][0] - dp[u][2]));
}
}
if(mn==INF){
dp[v][0] = INF;
}
else{
dp[v][0] += mn;
}
}
else{
int mn = INF;
for(int u : G[v]){
if(u==p)
continue;
if(dp[u][0] < INF && dp[u][2] < INF){
mn = min(mn, abs(dp[u][0] - dp[u][2]));
}
}
if(mn==INF){
dp[v][1] = INF;
}
else{
dp[v][1] += mn;
}
}
}
void upd2(int v, int p){
int cnt = 0;
bool C = 0;
for(int u : G[v]){
if(u == p)
continue;
C |= (dp[u][1] >= INF && dp[u][3] >= INF);
if(dp[u][1] < dp[u][3]){
dp[v][2] += dp[u][1];
}
else{
dp[v][2] += dp[u][3];
cnt++;
}
}
if(C){
dp[v][2] = INF;
dp[v][3] = INF;
return;
}
dp[v][2]++;
dp[v][3] = dp[v][2];
if((f[v] && cnt%2==0) || (!f[v] && cnt % 2)){
int mn = INF;
for(int u : G[v]){
if(u==p)
continue;
if(dp[u][1] < INF && dp[u][3] < INF){
mn = min(mn, abs(dp[u][1] - dp[u][3]));
}
}
if(mn==INF){
dp[v][3] = INF;
}
else{
dp[v][3] += mn;
}
}
else{
int mn = INF;
for(int u : G[v]){
if(u==p)
continue;
if(dp[u][1] < INF && dp[u][3] < INF){
mn = min(mn, abs(dp[u][1] - dp[u][3]));
}
}
if(mn==INF){
dp[v][2] = INF;
}
else{
dp[v][2] += mn;
}
}
}
void DFS(int v, int p = -1){
if(sz(G[v])==1 && p != -1){
if(f[v]){
dp[v][0] = INF, dp[v][1] = 0, dp[v][2] = 1, dp[v][3] = INF;
}
else{
dp[v][0] = 0, dp[v][1] = INF, dp[v][2] = INF, dp[v][3] = 1;
}
return;
}
for(int u : G[v])if(u != p){
DFS(u, v);
}
upd1(v, p);
upd2(v, p);
}
int32_t main(){
ios::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int n;
cin >> n;
for(int i =1;i < n ;i++){
int v, u;
cin >> v >> u ;
G[v].pb(u);
G[u].pb(v);
}
for(int i = 1; i <= n; i++)
cin >> f[i];
DFS(1);
int mn = min(dp[1][0], dp[1][2]);
if(mn >= INF)
KILL("impossible");
KILL(mn);
}
