#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 --;
vector<vector<ll>> dp2(2,vector<ll>(2));
vector<pair<ll,ll>> ops;
for(auto ch : adj[node]){
if(ch == par) continue;
ops.push_back({solve(ch,node,1,on[ch]^1) + 1 , solve(ch,node,0,on[ch])});
}
dp2[sz&1][color] = 0;
dp2[sz&1][!color] = 1e9;
for(int i=sz-1;i>=0;i--){
for(int j=0;j<2;j++){
dp2[i&1][j] = min(dp2[i+1&1][j]+ops[i].second , dp2[i+1&1][j^1] + ops[i].first);
}
}
ret = min(ret,dp2[0][0]);
}
if(pressed != 0){
int sz = adj[node].size();
if(node) sz --;
vector<vector<ll>> dp2(2,vector<ll>(2));
vector<pair<ll,ll>> ops;
for(auto ch : adj[node]){
if(ch == par) continue;
ops.push_back({solve(ch,node,1,on[ch]) + 1 , solve(ch,node,0,on[ch]^1)});
}
dp2[sz&1][color] = (pressed == 1 ? 0 : 1e9);
dp2[sz&1][!color] = (pressed == 2 ? 0 : 1e9);
for(int i=sz-1;i>=0;i--){
for(int j=0;j<2;j++){
dp2[i&1][j] = min(dp2[i+1&1][j]+ops[i].second , dp2[i+1&1][j^1] + ops[i].first);
}
}
ret = min(ret,dp2[0][0] + (pressed == 2));
}
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;
}
