#include <bits/stdc++.h>
using namespace std;
#define sz(v) (int) v.size()
#define pb push_back
#define popcount(x) __builtin_popcount(x)
using i64 = long long;
template<typename T>
using vec = vector<T>;
template<typename T>
void chmin(T &a,T b){ a = min(a,b); }
int n;
vec<vec<int>> adj;
vec<int> col;
i64 dp[100100][2][2];
void dfs(int node,int p){
for (auto x : adj[node]){
if (x == p) continue;
dfs(x,node);
}
vec<vec<i64>> currdp(2,vec<i64>(2,INT_MAX));
currdp[col[node]][0] = 0;
currdp[(col[node] +1) % 2][1] = 1;
for (auto x : adj[node]){
if (x == p) continue;
vec<vec<i64>> nxtdp(2,vec<i64>(2,INT_MAX));
for (int j = 0; j < 2; j++){
for (int k = 0; k < 2; k++){
chmin(nxtdp[j][k],currdp[j][k] + dp[x][k][0]);
chmin(nxtdp[j][k],currdp[(j+1) % 2][k] + dp[x][k][1]);
}
}
swap(currdp,nxtdp);
}
for (int j = 0; j < 2; j++){
for (int k = 0; k < 2; k++) dp[node][j][k] = currdp[j][k];
}
}
int main(){
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
cin >> n;
adj.assign(n,vec<int>());
col.assign(n,0);
for (int i = 0; i < n-1; i++){
int u,v; cin >> u >> v;
u--; v--;
adj[u].pb(v);
adj[v].pb(u);
}
for (int i = 0; i < n; i++) cin >> col[i];
for (int i = 0; i < n; i++){
for (int j = 0; j < 2; j++){
for (int k = 0; k < 2 ; k++){
dp[i][j][k] = INT_MAX;
}
}
}
dfs(0,-1);
// for (int i = 0; i < n; i++){
// for (int j = 0; j < 2; j++){
// for (int k = 0; k < 2; k++) cout << i << " " << j <<" " << k << ": " << dp[i][j][k] << endl;
// }
// }
i64 ans = min(dp[0][0][0],dp[0][0][1]);
if (ans == INT_MAX) cout << "impossible" << endl;
else cout << ans << endl;
return 0;
}
