#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
typedef long long ll;
using namespace __gnu_pbds;
using namespace std;
template <typename T>
using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
int M = 100'010;
vector<vector<int>> g(M), W(M, vector<int>(2, 0)), N(M, vector<int>(2, 0));
vector<int> val(M);
int inf = 1e7;
void dp_dfs(int v, int p) {
vector<int> taken(2), sum(2), zastap(2, inf);
int sasiad = 0;
for(auto u : g[v]) {
if(u==p) continue;
sasiad++;
dp_dfs(u, v);
for(int k : {0,1}) {
sum[k] += min(N[u][k], W[u][k]);
if(W[u][k] <= N[u][k]) taken[k]++;
zastap[k] = min(zastap[k], max(W[u][k],N[u][k]) - min(W[u][k],N[u][k]));
}
}
if(!sasiad) {
W[v][val[v]^1] = 1;
W[v][val[v]] = inf;
N[v][val[v]^1] = inf;
N[v][val[v]] = 0;
//for(int k : {0,1}) cout << v << " " << k << ": " << W[v][k] << " " << N[v][k] << "\n";
return;
}
int K1 = val[v]^(taken[0]%2);
N[v][K1] = sum[0]; N[v][K1^1] = sum[0]+zastap[0];
int K2 = val[v]^(taken[1]%2) ^ 1;
W[v][K2] = 1 + sum[1]; W[v][K2^1] = 1+sum[1]+zastap[1];
for(int k : {0,1}) {
W[v][k] = min(W[v][k], inf);
N[v][k] = min(N[v][k], inf);
//cout << v << " " << k << ": " << W[v][k] << " " << N[v][k] << "\n";
}
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0); cout.tie(0);
int n;
cin >> n;
for(int i=1; i<n; ++i) {
int a, b;
cin >> a >> b;
g[a].push_back(b);
g[b].push_back(a);
}
for(int i=1; i<=n; ++i) cin >> val[i];
int R = 1;
dp_dfs(R, 0);
int ans = min(W[R][0], N[R][0]);
if(ans>=inf) cout << "impossible\n";
else cout << ans << "\n";
return 0;
}
