#include <bits/stdc++.h>
#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2")
#define int long long
#define pii pair<int,int>
#define vi vector<int>
#define ff first
#define ss second
#define sp << " " <<
#define all(x) x.begin(),x.end()
#define big(x) ((int)(x.size()))
using namespace std;
const int MOD = 1e9+7, LIM = 1e6+1, inf = 2e17;
const int N = 1e5+1;
int dp[N][2][2];
vi edges[N],c(N);
int dfs(int node,int p,int press,int color) {
if (~dp[node][press][color]) return dp[node][press][color];
dp[node][press][color] = inf;
int f[2];
f[c[node]^press] = press,f[c[node]^press^1] = inf;
for (auto it : edges[node]) {
if (it == p) continue;
int a = dfs(it,node,0,press);
int b = dfs(it,node,1,press);
int f0 = f[0],f1 = f[1];
f[0] = min(f1+b,f0+a),f[1] = min(f0+b,f1+a);
}
for (int i = 0;i<2;i++) dp[node][press][i] = f[i];
return dp[node][press][color];
}
void solve() {
int n;
cin >> n;
memset(dp,-1,sizeof dp);
for (int i=1;i<n;i++) {
int a,b;
cin >> a >> b;
edges[a].push_back(b);
edges[b].push_back(a);
}
for (int i=1;i<=n;i++) cin >> c[i];
dfs(1,1,0,0),dfs(1,1,1,0);
int ans = min(dp[1][0][0],dp[1][1][0]);
if (ans >= 1e17) cout << "impossible\n";
else cout << ans << '\n';
}
signed main() {
ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef Dodi
freopen("in.txt", "r", stdin);
freopen("out.txt", "w", stdout);
#endif
int t = 1;
//cin >> t;
while (t --> 0) solve();
}
