#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
#define FOR(i, l, r, d) for(int i=(l); i<=(r); i+=(d))
#define szof(x) ((int)(x).size())
#define vi vector<int>
#define pii pair<int, int>
#define F first
#define S second
#define pb push_back
#define eb emplace_back
#define mkp make_pair
const int INF = 2147483647;
const int LNF = INF*INF;
const int MOD = 1000000007;
const int mod = 998244353;
const double eps = 1e-12;
//#pragma GCC optimize("Ofast")
//#pragma GCC optimize("unroll-loops")
const int MAX = 100010;
int n;
int val[MAX];
vi edge[MAX];
vi ch[MAX];
int dp[MAX][2][2]; // dp[v][v is 1][parent of v is switched]
void dfs(int v, int pv){
for(int i : edge[v]){
if(i == pv) continue;
ch[v].pb(i);
dfs(i, v);
}
}
void solve(int v){
dp[v][0][0] = dp[v][0][1] = dp[v][1][0] = dp[v][1][1] = INF;
for(int i : ch[v]){
solve(i);
}
int sz = szof(ch[v]);
//int new_val_v = val[v];
//int dp_sum = 0;
int opt[2], n_opt[2]; // dp_sum if new_val_v == 0 / 1
// dp[v][?][0] :
// all childs = 0 -> do not flip v
// pick dp[u][0][?] for all childs u
opt[val[v]] = 0;
opt[val[v] ^ 1] = INF;
FOR(i, 0, sz-1, 1){
int u = ch[v][i];
//new_val_v ^= state[i];
//dp_sum += dp[u][0][state[i]];
n_opt[0] = min(opt[0] + dp[u][0][0], opt[1] + dp[u][0][1]);
n_opt[1] = min(opt[1] + dp[u][0][0], opt[0] + dp[u][0][1]);
opt[0] = n_opt[0];
opt[1] = n_opt[1];
}
dp[v][0][0] = min(dp[v][0][0], opt[0]);
dp[v][1][0] = min(dp[v][1][0], opt[1]);
// dp[v][?][1] :
// all childs = 1 -> flip v
// pick dp[u][1][?] for all childs u
opt[val[v]] = 0;
opt[val[v] ^ 1] = INF;
FOR(i, 0, sz-1, 1){
int u = ch[v][i];
n_opt[0] = min(opt[0] + dp[u][1][0], opt[1] + dp[u][1][1]);
n_opt[1] = min(opt[1] + dp[u][1][0], opt[0] + dp[u][1][1]);
opt[0] = n_opt[0];
opt[1] = n_opt[1];
}
// flip v : upd. new_val_v, dp_sum
//new_val_v ^= 1;
//dp_sum += 1;
swap(opt[0], opt[1]);
opt[0] += 1;
opt[1] += 1;
dp[v][0][1] = min(dp[v][0][1], opt[0]);
dp[v][1][1] = min(dp[v][1][1], opt[1]);
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
cout.tie(0);
cin>>n;
FOR(i, 1, n-1, 1){
int u, v;
cin>>u>>v;
edge[u].pb(v);
edge[v].pb(u);
}
FOR(i, 1, n, 1){
cin>>val[i];
}
dfs(1, -1);
solve(1);
int res = min(dp[1][0][0], dp[1][0][1]);
if(res < INF) cout<<res<<'\n';
else cout<<"impossible"<<'\n';
return 0;
}
