#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
#define fi first
#define se second
#define vi vector<int>
#define vll vector<long long>
#define pii pair<int, int>
#define pll pair<long long, long long>
#define pb push_back
#define mp make_pair
#define eb emplace_back
#define all(x) (x).begin(), (x).end()
#define sz(x) (int)(x).size()
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
mt19937_64 rng64(chrono::steady_clock::now().time_since_epoch().count());
inline int rand(int l,int r){return uniform_int_distribution<int>(l, r)(rng);}
inline ll rand(ll l,ll r){return uniform_int_distribution<ll>(l, r)(rng64);}
#ifdef DEBUG
auto&operator<<(auto&o,pair<auto,auto>p){return o<<"("<<p.first<<", "<<p.second<<")";}
auto operator<<(auto&o,auto x)->decltype(x.end(),o){o<<"{";int i=0;for(auto e:x)o<<","+!i++<<e;return o<<"}";}
#define debug(X...)cerr<<"["#X"]: ",[](auto...$){((cerr<<$<<"; "),...)<<endl;}(X)
#else
#define debug(...){}
#endif
struct custom_hash {
static uint64_t splitmix64(uint64_t x) {
x += 0x9e3779b97f4a7c15;
x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
return x ^ (x >> 31);
}
size_t operator()(uint64_t x) const {
static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
return splitmix64(x + FIXED_RANDOM);
}
};
const int MAXN = 1e5 + 7;
const int INF = 1e9;
vi g[MAXN];
int a[MAXN];
int dp[MAXN][2][2];
int n;
void dfs(int v, int p){
for(auto u : g[v]){
if(u == p){
continue;
}
dfs(u, v);
}
dp[v][a[v]][0] = 0;
dp[v][1 - a[v]][1] = 1;
for(auto u : g[v]){
if(u == p){
continue;
}
vi xd = {INF, INF};
vector<vi> ndp;
ndp.pb(xd);
ndp.pb(xd);
for(int i = 0; i < 2; i++){
for(int j = 0; j < 2; j++){
ndp[(i + j) % 2][0] = min(ndp[(i + j) % 2][0], dp[v][j][0] + dp[u][0][i]);
ndp[(i + j) % 2][1] = min(ndp[(i + j) % 2][1], dp[v][j][1] + dp[u][1][i]);
}
}
for(int i = 0; i < 2; i++){
for(int j = 0; j < 2; j++){
dp[v][i][j] = ndp[i][j];
}
}
}
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);
cout.tie(NULL);
cin >> n;
for(int i = 1; i < n; i++){
int u, v;
cin >> u >> v;
g[u].pb(v);
g[v].pb(u);
}
for(int i = 1; i <= n; i++){
cin >> a[i];
for(int j = 0; j < 2; j++){
for(int y = 0; y < 2; y++){
dp[i][j][y] = INF;
}
}
}
dfs(1, 1);
if(dp[1][0][1] == INF && dp[1][0][0] == INF){
cout << "impossible\n";
}else{
cout << min(dp[1][0][1], dp[1][0][0]) << '\n';
}
return 0;
}
