// Born_To_Laugh - Hughie Do
#include <bits/stdc++.h>
#define alle(AC) AC.begin(), AC.end()
#define fi first
#define se second
using namespace std;
typedef long long ll;
[[maybe_unused]] const ll MOD = 998244353, INF = 1e9 + 7;
const int maxn = 1e5 + 10;
int n;
int cc[maxn];
vector<int> adj[maxn];
ll dp[maxn][2][2];
void dfs(int a, int par){
if(cc[a]){
dp[a][1][0] = 1;
dp[a][0][1] = 0;
dp[a][1][1] = INF;
dp[a][0][0] = INF;
}
else{
dp[a][1][0] = INF;
dp[a][0][1] = INF;
dp[a][1][1] = 1;
dp[a][0][0] = 0;
}
for(auto &elm: adj[a]){
if(elm == par)continue;
dfs(elm, a);
int nxt[2][2];
for(int i=0; i<2; ++i){
for(int j=0; j<2; ++j){
nxt[i][j] = min({
dp[a][i][0] + dp[elm][j][i],
dp[a][i][1] + dp[elm][1 ^ j][i]
});
}
}
for(int i=0; i<2; ++i){
for(int j=0; j<2; ++j){
dp[a][i][j] = nxt[i][j];
}
}
}
}
void solve(){
cin >> n;
for(int i=1; i<n; ++i){
int a, b;cin >> a >> b;
adj[a].push_back(b);
adj[b].push_back(a);
}
for(int i=1; i<=n; ++i)cin >> cc[i];
dfs(1, -1);
int ans = min(dp[1][1][0], dp[1][0][0]);
if(ans > maxn){
cout << "impossible" << '\n';
}
else cout << ans << '\n';
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
solve();
}
