#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define fi first
#define se second
#define endl "\n"
#define pii pair<ll,ll>
#define pb push_back
#define vi vector<ll>
#define pque priority_queue
#define pqueg priority_queue<ll,vector<ll>,greater<ll>>
#define que queue<ll>
#define FOR(m,i,n) for(int i=(m); i<=(n); i++)
#define FORM(m,i,n) for(int i=(m); i>=(n); i--)
ll n,u,v,ans;
vector<ll> adj[100100];
bool vis[100100];
ll a[100100];
void dfs(ll x) {
vis[x] = true;
for(auto i : adj[x]) {
if(!vis[i]) {
if(a[i] == 1) {
a[min(n,i+1)] = a[min(n,i+1)] ^ 1 ^ 0;
a[min(n,i+2)] = a[min(n,i+2)] ^ 1 ^ 0;
ans++;
}
dfs(i);
}
}
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
FOR(1,i,n-1) {
cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
a[0] = -1;
FOR(1,i,n) {
cin >> a[i];
}
dfs(1);
FOR(1,i,a[i]) {
if(a[i] == 1) {
cout << "impossible" << endl;
return 0;
}
}
cout << ans << endl;
}
