#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("O3,unroll-loops")
// #pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")
// Ofast, O0, O1, O2, O3, unroll-loops, fast-math, trapv
typedef long long ll;
typedef pair<ll, ll> pll;
typedef pair<int, int> pii;
#define Mp make_pair
#define sep ' '
#define endl '\n'
#define F first
#define S second
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define kill(res) cout << res << '\n', exit(0);
#define set_dec(x) cout << fixed << setprecision(x);
#define fast_io ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define file_io freopen("input.txt", "r", stdin) ; freopen("output.txt", "w", stdout);
const ll N = 1e5 + 50;
const ll inf = 1e18;
ll n, col[N], dp[N][2][2], tmp[2][2];
vector<int> adj[N];
void dfs(int v, int p = 0){
for(int i = 0; i < 2; i++){
for(int j = 0; j < 2; j++){
dp[v][i][j] = inf;
}
}
dp[v][col[v]][0] = 0;
dp[v][1-col[v]][1] = 1;
for(int u: adj[v]){
if(u == p) continue;
dfs(u, v);
for(int i = 0; i < 2; i++){
for(int j = 0; j < 2; j++){
tmp[i][j] = inf;
}
}
for(int i = 0; i < 2; i++){
for(int j = 0; j < 2; j++){
for(int k = 0; k < 2; k++){
tmp[i][j] = min(tmp[i][j], dp[u][j][k] + dp[v][i^k][j]);
}
}
}
for(int i = 0; i < 2; i++){
for(int j = 0; j < 2; j++){
dp[v][i][j] = tmp[i][j];
}
}
}
}
int main(){
fast_io;
cin >> n;
int u, v;
for(int i = 1; i < n; i++){
cin >> u >> v;
adj[u].pb(v);
adj[v].pb(u);
}
for(int i = 1; i <= n; i++) cin >> col[i];
dfs(1);
ll res = min(dp[1][0][0], dp[1][0][1]);
if(res >= inf) kill("impossible");
cout << res;
}
