#include<bits/stdc++.h>
using namespace std;
#define int long long
const int N = 100000;
const int INF = 1e11;
vector<int> adj[N];
int dp[4][N]; //0 - I don't toggle. 1 - I toggle. 2, 3 - same but the camera is not off.
int initial[N];
void get0(int x, int p){
if(initial[x] == 0){
int ans = 0;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[0][node];
change.push_back(dp[1][node] - dp[0][node]);
}
}
sort(change.begin(), change.end());
int mn = 0;
int curr = 0;
for(int i = 0; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[0][x] = ans + mn;
}else{
dp[0][x] = INF;
int ans = 0;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[0][node];
change.push_back(dp[1][node] - dp[0][node]);
}
}
if(change.empty()) return;
sort(change.begin(), change.end());
int mn = change[0];
int curr = change[0];
for(int i = 1; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[0][x] = ans + mn;
}
}
void get1(int x, int p){
if(initial[x] == 1){
int ans = 1;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[2][node];
change.push_back(dp[3][node] - dp[2][node]);
}
}
sort(change.begin(), change.end());
int mn = 0;
int curr = 0;
for(int i = 0; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[1][x] = ans + mn;
}else{
dp[1][x] = INF;
int ans = 1;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[2][node];
change.push_back(dp[3][node] - dp[2][node]);
}
}
if(change.empty()) return;
sort(change.begin(), change.end());
int mn = change[0];
int curr = change[0];
for(int i = 1; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[1][x] = ans + mn;
}
}
void get2(int x, int p){
if(initial[x] == 1){
int ans = 0;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[0][node];
change.push_back(dp[1][node] - dp[0][node]);
}
}
sort(change.begin(), change.end());
int mn = 0;
int curr = 0;
for(int i = 0; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[2][x] = ans + mn;
}else{
dp[2][x] = INF;
int ans = 0;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[0][node];
change.push_back(dp[1][node] - dp[0][node]);
}
}
if(change.empty()) return;
sort(change.begin(), change.end());
int mn = change[0];
int curr = change[0];
for(int i = 1; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[2][x] = ans + mn;
}
}
void get3(int x, int p){
if(initial[x] == 0){
int ans = 1;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[2][node];
change.push_back(dp[3][node] - dp[2][node]);
}
}
sort(change.begin(), change.end());
int mn = 0;
int curr = 0;
for(int i = 0; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[3][x] = ans + mn;
}else{
dp[3][x] = INF;
int ans = 1;
vector<int> change;
for(int node : adj[x]){
if(node != p){
ans += dp[2][node];
change.push_back(dp[3][node] - dp[2][node]);
}
}
if(change.empty()) return;
sort(change.begin(), change.end());
int mn = change[0];
int curr = change[0];
for(int i = 1; i + 1 < (int)change.size(); i += 2){
curr += change[i] + change[i+1];
mn = min(mn, curr);
}
dp[3][x] = ans + mn;
}
}
void DFS(int x, int p){
for(int node : adj[x]){
if(node != p) DFS(node, x);
}
get0(x, p);
get1(x, p);
get2(x, p);
get3(x, p);
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int n; cin >> n;
for(int i = 0; i < n - 1; i++){
int a, b; cin >> a >> b; a--; b--;
adj[a].push_back(b);
adj[b].push_back(a);
}
for(int i = 0; i < n; i++) cin >> initial[i];
DFS(0, -1);
if(min(dp[0][0], dp[1][0]) >= 1e6) cout << "impossible\n";
else cout << min(dp[0][0], dp[1][0]) << "\n";
}
