#include <bits/stdc++.h>
#define int long long
#define pb push_back
using namespace std;
const int N = 1e5 + 5,inf=1e9;
vector <int> v[N];
int c[N];
int dp[4][N][4];
void dfs(int x,int par){
int valblack = 0,valblack0 = inf,parblack = 1;
int valwhite = 0,valwhite0 = inf,parwhite = 0;
for (int to: v[x]){
if (to == par) continue;
dfs(to,x);
//black
valblack += min(dp[0][to][1],dp[0][to][0]);
if (min(dp[0][to][1],dp[0][to][0]) == dp[0][to][1]) parblack++;
valblack0 = min(valblack0, abs(dp[0][to][1] - dp[0][to][0]));
//white
valwhite += min(dp[1][to][1],dp[1][to][0]);
if (min(dp[1][to][1],dp[1][to][0]) == dp[1][to][1]) parwhite++;
valwhite0 = min(valwhite0, abs(dp[1][to][1] - dp[1][to][0]));
}
parblack%=2;
parwhite%=2;
int color = 1 - c[x];
int oppcolor = c[x];
//x-ზე ოპერაციას ვაკეთებთ
if (parblack){
dp[oppcolor][x][1] = min(dp[oppcolor][x][1],valblack + 1);
dp[color][x][1] = min(dp[color][x][1],valblack + valblack0 + 1);
}else{
dp[color][x][1] = min(dp[color][x][1],valblack + 1);
dp[oppcolor][x][1] = min(dp[oppcolor][x][1],valblack + valblack0 + 1);
}
//x-ზე ოპერაციას არ ვაკეთებთ
if (parwhite){
dp[oppcolor][x][0] = min(dp[oppcolor][x][0],valwhite);
dp[color][x][0] = min(dp[color][x][0],valwhite + valwhite0);
}else{
dp[color][x][0] = min(dp[color][x][0],valwhite);
dp[oppcolor][x][0] = min(dp[oppcolor][x][0],valwhite + valwhite0);
}
}
signed main (){
ios_base::sync_with_stdio(0),cin.tie(NULL),cout.tie(NULL);
int n;
cin>>n;
for (int i = 1; i < n; i++){
int a,b;
cin>>a>>b;
v[a].pb(b);
v[b].pb(a);
}
for (int i = 1; i <= n; i++)
cin>>c[i]; //c[i] = 0 -> i-წვერო თეთრი ფერია
for (int i = 1; i <= n; i++)
for (int j = 0; j < 2; j++)
for (int p = 0; p < 2; p++)
dp[p][i][j] = inf;
//j ნიშნავს i წვეროზე ოპერაციას ვაკეთებ თუ არა
dfs(1,1);
int ans = min(dp[1][1][1],dp[1][1][0]);
if (ans == inf) cout<<"impossible";
else cout<<ans;
}
