#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define endl '\n'
vector<vector<int>> adj;
vector<int> on;
const int N = 1e5+5;
ll dp[N][3][2];
ll solve(int node,int par,int pressed,bool color){
ll &ret = dp[node][pressed][color];
if(~ret) return ret;
ret = 1e9;
if(pressed != 1){
int sz = adj[node].size();
if(node) sz --;
for(int mask=0;mask<(1<<sz);mask++){
bool changed = __builtin_popcount(mask) & 1;
if((color^changed)) continue;
ll c = 0;
int idx = 0;
for(auto ch : adj[node]){
if(ch == par) continue;
if(((mask>>idx)&1)) c += solve(ch,node,1,on[ch]^1) + 1;
else c += solve(ch,node,0,on[ch]);
idx ++;
}
ret = min(ret,c);
}
}
if(pressed != 0){
int sz = adj[node].size();
if(node) sz --;
for(int mask=0;mask<(1<<sz);mask++){
bool changed = __builtin_popcount(mask) & 1;
if(pressed == 1 && (color^changed)) continue;
if(pressed == 2 && !((color^changed))) continue;
ll c = 0;
int idx = 0;
for(auto ch : adj[node]){
if(ch == par) continue;
if(((mask>>idx)&1)) c += solve(ch,node,1,on[ch]) + 1;
else c += solve(ch,node,0,on[ch]^1);
idx ++;
}
if(pressed == 2) c ++;
ret = min(ret,c);
}
}
return ret;
}
void solve(){
int n;
cin>>n;
adj.resize(n);
for(int i=0;i<n-1;i++){
int a,b;
cin>>a>>b;
a -- , b --;
adj[a].push_back(b);
adj[b].push_back(a);
}
on.resize(n);
for(int i=0;i<n;i++){
int x;cin>>x;
if(x) on[i] = 1;
}
memset(dp,-1,sizeof(dp));
ll ans = solve(0,n,2,on[0]);
if(ans >= 1e9) cout<<"impossible"<<endl;
else cout<<ans<<endl;
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int t=1;
//cin>>t;
while(t--){
solve();
}
return 0;
}
