#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define ll long long
#define pb push_back
#define ii pair<int,int>
const int MAXN=1e5+5;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int dp[MAXN][2];
vector<int> tab(MAXN);
vector<int> adj[MAXN];
bool toggle[MAXN][2];
void dfs(int x,int p){
if(adj[x].size()==1&&p!=-1){
if(tab[x]==1){
dp[x][1]=0;
dp[x][0]=1;
toggle[x][1]=0;
toggle[x][0]=1;
}else{
dp[x][0]=0;
dp[x][1]=1;
toggle[x][0]=0;
toggle[x][1]=1;
}
return;
}
toggle[x][1]=1^tab[x];
toggle[x][0]=tab[x];
int one=0;
int zero=0;
for(auto u:adj[x]){
if(u==p) continue;
dfs(u,x);
one+=dp[u][1];
zero+=dp[u][0];
toggle[x][0]^=toggle[u][0];
toggle[x][1]^=toggle[u][1];
}
if(toggle[x][0]){
dp[x][1]=zero;
}else dp[x][0]=zero;
if(toggle[x][1]){
dp[x][1]=min(one+1,dp[x][1]);
}else dp[x][0]=min(dp[x][0],one+1);
toggle[x][1]=1^tab[x];
toggle[x][0]=tab[x];
//cout <<x<<" "<<dp[x][0]<<" "<<dp[x][1]<<endl;
return;
}
int main(){
int n;
cin>>n;
for (int i = 0; i < n; ++i)
{
dp[i][0]=dp[i][1]=1e9;
}
for (int i = 0; i < n-1; ++i)
{
int x,y;
cin>>x>>y;
x--;y--;
adj[x].pb(y);
adj[y].pb(x);
}
for (int i = 0; i < n; ++i)
{
cin>>tab[i];
}
dfs(0,-1);
if(dp[0][0]>=1e9) cout <<"impossible"<<endl;
else cout <<dp[0][0]<<endl;
}
