#include<iostream>
#include<vector>
#include<algorithm>
#include<iomanip>
using namespace std;
#define vi vector<int>
#define vvi vector<vi>
int inf = 1e8;
struct point {
int a = inf, b = inf, c = inf, d = inf; // no flip on, flip on, no flip off, flip off
};
vi O;
vvi Adj;
point oink(int p, int dfp) {
if (Adj[p].size() == 1 && Adj[p][0] == dfp) {
if (O[p]) return {0, inf, inf, 1};
return {inf, 1, 0, inf};
}
bool q = 1;
point l;
for (auto i : Adj[p]) {
if (i == dfp) continue;
point o = oink(i, p);
//cerr << "o: " << o.a << ", " << o.b << ", " << o.c << ", " << o.d << "; p:" << i+1 << endl;
if (q) {
l = o;
q = 0;
continue;
}
point ln;
ln.a = min(min(l.a + o.a, l.b + o.b), inf);
ln.b = min(min(l.a + o.b, l.b + o.a), inf);
ln.c = min(min(l.c + o.c, l.d + o.d), inf);
ln.d = min(min(l.c + o.d, l.d + o.c), inf);
l = ln;
}
//cerr << l.a << " " << l.b << " " << l.c << " " << l.d << "; p: " << p+1 << "; dfp: " << dfp+1 << endl;
if (O[p]) return {l.c, l.b+1, l.d, l.a+1};
return {l.d, l.a+1, l.c, l.b+1};
}
int main() {
ios::sync_with_stdio(); cin.tie(0);
int n; cin >> n;
Adj.resize(n);
for (int i = 0; i < n-1; i++) {
int a, b; cin >> a >> b; a--; b--;
Adj[a].push_back(b);
Adj[b].push_back(a);
}
O.resize(n);
for (auto &i : O) cin >> i;
point ans = oink(0, -1);
int a = min(ans.c, ans.d);
if (a >= inf) cout << "impossible\n";
else cout << a << "\n";
}
