#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int>vi;
#define pb push_back
#define sz(v) (int)v.size()
#define all(x) begin(x),end(x)
#define FOR(i,a,b) for(int i=a; i<b; i++)
#define ROF(i,a,b) for(int i=b-1; i>=a; i--)
void IO(){
#ifdef LOCAL
freopen("input.txt","r",stdin);
freopen("output.txt","w",stdout);
#endif
}
//-----------------------------------------
void ckmin(int &x, int y){
x=min(x,y);
}
const int INF=1e9;
const int MX=1e5+10;
int N,memo[MX][2][2];
vi a(MX),adj[MX];
int solve(int u, int p, int i, int j){
int &ind=memo[u][i][j];
if(ind!=-1) return ind;
vi val(2,0); val[1]=INF;
for(int v: adj[u]) if(v!=p){
vi new_val(2,INF);
ckmin(new_val[0],val[1]+solve(v,u,j,1));
ckmin(new_val[1],val[0]+solve(v,u,j,1));
ckmin(new_val[0],val[0]+solve(v,u,j,0));
ckmin(new_val[1],val[1]+solve(v,u,j,0));
val.assign(all(new_val));
}
int state=a[u]^i^j,ans=j+val[state];
return ind=ans;
}
int main(){
IO();
cin>>N;
FOR(i,0,N-1){
int u,v; cin>>u>>v;
adj[u].pb(v);
adj[v].pb(u);
}
FOR(i,1,N+1) cin>>a[i];
memset(memo,-1,sizeof(memo));
int ans=min(solve(1,1,0,1),solve(1,1,0,0));
if(ans>=INF) cout << "impossible" << endl;
else cout << ans << endl;
}
