#include <iostream>
#include <vector>
#define endl '\n'
#define pb push_back
#define ll long long
using namespace std;
const int MAXN = 1e5+100;
const ll INF = 1e6+109;
int n;
bool a[MAXN], used[MAXN];
vector<int> adj[MAXN];
ll dp[MAXN][2][2];
ll min(ll a, ll b){
return (a < b)? a : b;
}
void dfs(int v){
used[v] = 1;
ll cdp[2][2];
ll tmp[2][2];
bool flg = true;
for(int u : adj[v]){
if(used[u]) continue;
dfs(u);
if(flg){
for(int i=0; i<2; i++){
for(int j=0; j<2; j++){
cdp[i][j] = dp[u][i][j];
tmp[i][j] = INF;
}
}
}
else{
for(int i=0; i<2; i++){
tmp[i][0] = min(tmp[i][0], cdp[i][0] + dp[u][i][0]);
tmp[i][0] = min(tmp[i][0], cdp[i][1] + dp[u][i][1]);
tmp[i][1] = min(tmp[i][1], cdp[i][0] + dp[u][i][1]);
tmp[i][1] = min(tmp[i][1], cdp[i][1] + dp[u][i][0]);
/*
if(v == 1){
cout << "A: "<< endl;
for(int i=0; i<2; i++){
for(int j=0; j<2; j++) cout << tmp[i][j] << ' ';
cout << endl;
}
}
*/
}
}
for(int i=0; i<2; i++){
for(int j=0; j<2; j++){
if(!flg) cdp[i][j] = tmp[i][j];
tmp[i][j] = INF;
}
}
flg = false;
}
for(int i=0; i<2; i++){
for(int j=0; j<2; j++){
if(a[v]) dp[v][i][j] = cdp[j][i^j^1] + j;
else dp[v][i][j] = cdp[j][i^j] + j;
}
}
if(flg){
if(a[v]){
dp[v][1][0] = 0;
dp[v][0][1] = 1;
dp[v][1][1] = INF;
dp[v][0][0] = INF;
}
else{
dp[v][0][0] = 0;
dp[v][1][1] = 1;
dp[v][1][0] = INF;
dp[v][0][1] = INF;
}
}
}
int main(){
ios::sync_with_stdio(0);cin.tie(0);
cin >> n;
for(int i=1; i<n; i++){
int x, y;
cin >> x >> y;
x--;y--;
adj[x].pb(y);
adj[y].pb(x);
}
for(int i=0; i<n; i++) cin >> a[i];
dfs(0);
if(min(dp[0][0][0], dp[0][0][1]) >= INF) cout << "impossible" << endl;
else cout << min(dp[0][0][0], dp[0][0][1]) << endl;
return 0;
}
