//Sylwia Sapkowska
#include <bits/stdc++.h>
#pragma GCC optimize("O3", "unroll-loops")
using namespace std;
void __print(int x) {cerr << x;}
void __print(long long x) {cerr << x;}
void __print(long double x) {cerr << x;}
void __print(char x) {cerr << '\'' << x << '\'';}
void __print(const char *x) {cerr << '\"' << x << '\"';}
void __print(const string &x) {cerr << '\"' << x << '\"';}
void __print(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V>
void __print(const pair<T, V> &x) {cerr << '{'; __print(x.first); cerr << ", "; __print(x.second); cerr << '}';}
template<typename T>
void __print(const T &x) {int f = 0; cerr << '{'; for (auto &i: x) cerr << (f++ ? ", " : ""), __print(i); cerr << "}";}
void _print() {cerr << "]\n";}
template <typename T, typename... V>
void _print(T t, V... v) {__print(t); if (sizeof...(v)) cerr << ", "; _print(v...);}
#ifdef LOCAL
#define debug(x...) cerr << "[" << #x << "] = ["; _print(x)
#else
#define debug(x...)
#endif
#define int long long
typedef pair<int, int> T;
const int oo = 1e18, oo2 = 1e9+7, K = 30;
const int mod = 998244353;
void solve(){
int n; cin >> n;
vector<vector<int>>g(n+1);
for (int i = 1; i<n; i++){
int a, b; cin >> a >> b;
g[a].emplace_back(b);
g[b].emplace_back(a);
}
vector<int>state(n+1);
for (int i = 1; i<=n; i++) cin >> state[i];
vector dp(n+1, vector<vector<int>>(2, vector<int>(2, oo)));
//dp[v][2][2] = answer for subtree of v if we dont/do change the switch v-th camera and the state of v is off/on
function<void(int, int)>dfs = [&](int v, int pa){
for (int rep = 0; rep < 2; rep++){
dp[v][rep][state[v] ^ rep] = rep;
dp[v][rep][state[v] ^ rep ^ 1] = oo;
}
for (auto x: g[v]){
if (x == pa) continue;
dfs(x, v);
for (int rep = 0; rep < 2; rep++){
int dp0 = dp[v][rep][0];
int dp1 = dp[v][rep][1];
dp[v][rep][0] = min({oo, dp0 + dp[x][0][rep], dp1 + dp[x][1][rep]});
dp[v][rep][1] = min({oo, dp0 + dp[x][1][rep], dp1 + dp[x][0][rep]});
}
}
};
dfs(1, 1);
int ans = min(dp[1][1][0], dp[1][0][0]);
if (ans == oo) cout << "impossible\n";
else cout << ans << "\n";
}
int32_t main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
int t = 1;
// cin >> t;
while (t--) solve();
return 0;
}
