#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());
ll dp[MAXN][2][2];
vector<int> tab(MAXN);
vector<int> adj[MAXN];
void dfs(int x,int p){
for (int i = 0; i < 2; ++i)
{
dp[x][i][tab[x]^i]=i;
dp[x][i][tab[x]^i^1]=1e9;
}
for(auto u:adj[x]){
if(u==p) continue;
dfs(u,x);
for (int i = 0; i < 2; ++i)
{
ll cur1=dp[x][i][0];
ll cur2=dp[x][i][1];
dp[x][i][0]=min(1ll*1000000000,min(cur1+dp[u][0][i],cur2+dp[u][1][i]));
dp[x][i][1]=min(1ll*1000000000,min(cur2+dp[u][0][i],cur1+dp[u][1][i]));
}
}
//cout <<x<<" "<<dp[x][0][0]<<" "<<dp[x][0][1]<<" "<<dp[x][1][0]<<" "<<dp[x][1][1]<<endl;
return;
}
int main(){
int n;
cin>>n;
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(min(dp[0][0][0],dp[0][1][0])>=1e9) cout <<"impossible"<<endl;
else cout <<min(dp[0][0][0],dp[0][1][0])<<endl;
}
