#include <bits/stdc++.h>
using namespace std;
#define int long long
#define ld long double
#define show(x,y) //cout << y << " " << #x << endl;
#define show2(x,y,i,j) //cout << y << " " << #x << " " << j << " " << #i << endl;
#define show3(x,y,i,j,p,q) //cout << y << " " << #x << " " << j << " " << #i << " " << q << " " << #p << endl;
#define show4(x,y) for(auto it:x) cout << it << " "; cout << #y << endl;
typedef pair<int,int>pii;
typedef pair<pii,pii>pi2;
int n;
vector<int>adj[100005];
int arr[100005];
int dp[2][2][100005];
void dfs(int index, int par){
array<int,2>counter;
array<int,2>change;
array<int,2>cnt;
change={(int)1e14,(int)1e14};
cnt={0,0};
counter={0,0};
for(auto it:adj[index]){
if(it==par) continue;
dfs(it,index);
for(int x=0;x<2;x++){
if(dp[0][x][it]<dp[1][x][it]){
//no toggle is better
counter[x]+=dp[0][x][it];
change[x]=min(change[x],dp[1][x][it]-dp[0][x][it]);
}
else{
//toggle is better
counter[x]+=dp[1][x][it];
change[x]=min(change[x],dp[0][x][it]-dp[1][x][it]);
cnt[x]++;
}
}
}
//no toggle remain 0
dp[0][0][index]=counter[0]+ ((cnt[0]^arr[index])%2? change[0]:0);
dp[0][1][index]=counter[0]+((cnt[0]^arr[index])%2==0?change[0]:0);
dp[1][0][index]=1+counter[1]+((cnt[1]^arr[index])%2==0?change[1]:0);
dp[1][1][index]=1+counter[1]+((cnt[1]^arr[index])%2?change[1]:0);
}
void solve(){
cin >> n;
int temp,temp2;
for(int x=0;x<n-1;x++){
cin >> temp >> temp2;
adj[temp].push_back(temp2);
adj[temp2].push_back(temp);
}
for(int x=1;x<=n;x++){
cin >> arr[x];
}
dfs(1,-1);
int hold=min(dp[1][0][1],dp[0][0][1]);
if(hold>1e13){
cout << "impossible\n";
}
else cout << hold;
}
int32_t main(){
ios::sync_with_stdio(0);
cin.tie(0);
//freopen("redistricting.in", "r", stdin);
//freopen("redistricting.out", "w", stdout);
int t=1;
//cin >> t;
while(t--){
solve();
}
}
