#include<bits/stdc++.h>
using namespace std;
#define int long long
constexpr int INF=3e18;
signed main(){
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n; cin>>n;
vector<int> adj[n];
for(int i=0;i<n-1;++i){
int u,v; cin>>u>>v; --u,--v;
adj[u].emplace_back(v);
adj[v].emplace_back(u);
}
int f[n]; for(int i=0;i<n;++i){
cin>>f[i];
}
int pa[n],ord[n],_ord=0;
stack<int> st;
st.emplace(0); pa[0]=-1;
while(st.size()){
int u=st.top(); st.pop();
ord[_ord++]=u;
for(auto&v:adj[u]) if(v!=pa[u]){
pa[v]=u;
st.emplace(v);
}
}
int dp[n][2][2];
for(auto&a:dp) for(auto&b:a) for(auto&c:b) c=INF;
for(int k=n-1;k>=0;--k){
int u=ord[k];
for(int i:{0,1}) for(int j:{0,1}){
int res=j,ce=0,co=INF;
bool c=true;
for(auto&v:adj[u]) if(v!=pa[u]){
if(dp[v][j][0]>=INF && dp[v][j][1]>=INF){
c=false;
break;
}
if(dp[v][j][0]>=INF){
res+=dp[v][j][1];
swap(ce,co);
}else if(dp[v][j][1]>=INF){
res+=dp[v][j][0];
}else{
res+=dp[v][j][0];
int tce=ce,tco=co;
ce=min(tce,tco+dp[v][j][1]-dp[v][j][0]);
co=min(tco,tce+dp[v][j][1]-dp[v][j][0]);
}
}
if(!c) continue;
int req=f[u]^i^j,inc=(req?co:ce);
if(inc<INF) dp[u][i][j]=res+inc;
}
}
int ans=min(dp[0][0][0],dp[0][0][1]);
if(ans>n) cout<<"impossible\n";
else cout<<ans<<'\n';
return 0;
}
