#include<bits/stdc++.h>
#define ll long long
#define pii pair<int,int>
#define pll pair<ll,ll>
#define plx pair<ll,int>
#define f first
#define s second
#define pb push_back
#define all(x) x.begin(),x.end()
#define vi vector<int>
#define vvi vector<vi>
#define pp pair<ll,int>
#define ub(x,i) upper_bound(all(x),i)-x.begin()
using namespace std;
const int mxn=1e5+5;
const int inf=1e9;
vector<int>g[mxn];
int col[mxn]{0};
ll dp[mxn][4]{0};
void sol(int u,int p){
dp[u][0]=dp[u][1]=dp[u][2]=dp[u][3]=1e9;
int sz1=0,sz2=0;
ll sm1=0,sm2=0;
ll mn1=1e9,mn2=1e9;
for(auto v:g[u]){
if(v==p)continue;
sol(v,u);
if(dp[v][0]>dp[v][2]){
sm1+=dp[v][2];
sz1++;
mn1=min(mn1,dp[v][0]-dp[v][2]);
}else sm1+=dp[v][0],mn1=min(mn1,dp[v][2]-dp[v][0]);
if(dp[v][1]>dp[v][3]){
sm2+=dp[v][3];
sz2++;
mn2=min(mn2,dp[v][1]-dp[v][3]);
}else sm2+=dp[v][1],mn2=min(mn2,dp[v][3]-dp[v][1]);
}
if(g[u].size()==1&&u!=1){
if(!col[u])dp[u][0]=0,dp[u][1]=1e9,dp[u][2]=1e9,dp[u][3]=1;
else dp[u][0]=inf,dp[u][1]=0,dp[u][2]=1,dp[u][3]=inf;
return;
}
//if(u==3)cout<<sm1<<' '<<sm2<<' '<<sz1<<' '<<sz2<<'\n';
if(!col[u]){
if(sz1&1)dp[u][0]=sm1+mn1,dp[u][1]=sm1;
else dp[u][0]=sm1,dp[u][1]=sm1+mn1;
if(sz2&1)dp[u][2]=1+sm2,dp[u][3]=1+sm2+mn2;
else dp[u][2]=1+sm2+mn2,dp[u][3]=1+sm2;
}
else {
if(!(sz1&1))dp[u][0]=sm1+mn1,dp[u][1]=sm1;
else dp[u][0]=sm1,dp[u][1]=sm1+mn1;
if(!(sz2&1))dp[u][2]=1+sm2,dp[u][3]=1+sm2+mn2;
else dp[u][2]=1+sm2+mn2,dp[u][3]=1+sm2;
}
}
int main(){
ios_base::sync_with_stdio(0);cin.tie(0);
int n;cin>>n;
for(int i=1;i<=n-1;i++){
int a,b;cin>>a>>b;g[a].pb(b);g[b].pb(a);
}for(int i=1;i<=n;i++)cin>>col[i];sol(1,1);
ll rs=min(dp[1][0],dp[1][2]);
if(rs>=1e9)cout<<"impossible";
else cout<<rs;
}
