#include <bits/stdc++.h>
#define DIM 200010
#define INF 10000000
using namespace std;
int dp[DIM][2][2],v[DIM],aux[2][2];
vector <int> L[DIM];
int n,i,x,y;
void dfs (int nod, int tata){
int ok = 0;
for (auto vecin : L[nod])
if (vecin != tata){
ok = 1;
dfs (vecin,nod);
}
if (!ok){
dp[nod][v[nod]][0] = 0;
dp[nod][1-v[nod]][1] = 1;
} else {
/// caz 1: nu dau switch in nod
dp[nod][v[nod]][0] = 0;
dp[nod][1-v[nod]][1] = 1;
for (auto vecin : L[nod]){
if (vecin == tata)
continue;
for (int i=0;i<2;i++)
for (int j=0;j<2;j++)
aux[i][j] = INF;
aux[0][0] = min (dp[nod][0][0] + dp[vecin][0][0], dp[nod][1][0] + dp[vecin][0][1]);
aux[0][1] = min (dp[nod][0][1] + dp[vecin][1][0], dp[nod][1][1] + dp[vecin][1][1]);
aux[1][0] = min (dp[nod][1][0] + dp[vecin][0][0], dp[nod][0][0] + dp[vecin][0][1]);
aux[1][1] = min (dp[nod][1][1] + dp[vecin][1][0], dp[nod][0][1] + dp[vecin][1][1]);
for (int i=0;i<2;i++)
for (int j=0;j<2;j++)
dp[nod][i][j] = min(INF,aux[i][j]);
}
}
}
int main (){
//ifstream cin ("date.in");
//ofstream cout ("date.out");
cin>>n;
for (i=1;i<n;i++){
cin>>x>>y;
L[x].push_back(y);
L[y].push_back(x);
}
for (i=1;i<=n;i++){
cin>>v[i];
dp[i][0][0] = dp[i][0][1] = dp[i][1][0] = dp[i][1][1] = INF;
}
dfs (1,0);
int sol = min(dp[1][0][0],dp[1][0][1]);
if (sol == INF)
cout<<"impossible";
else cout<<sol;
return 0;
}
