#include <bits/stdc++.h>
using namespace std;
#define F0R(i, n) for (ll i = 0; i < n;i++)
#define FOR(i, j, n) for (ll i = j;i < n;i++)
using ll = long long;
template<typename T>
using V = vector<T>;
using vi = V<ll>;
constexpr ll INF = 1e9 + 7;
using pi = pair<ll,ll>;
using vvi = V<vi>;
vvi g;
V<vvi> dp;
vi state;
void solve(ll x, ll p) {
vi child;
for (auto y : g[x]) {
if (y == p) continue;
child.push_back(y);
solve(y, x);
}
F0R(i, 2) {
F0R(j, 2) {
if (child.empty()) {
dp[x][1][1] = 1;
dp[x][0][0] = 0;
continue;
}
ll best = INF;
vi sDp(2, INF); sDp[0] = 0;
for (auto y : child) {
vi newDp(2, INF);
F0R(k, 2) {
F0R(l, 2) {
newDp[k] = min(newDp[k], dp[y][(j + state[y]) % 2][l] + sDp[(k + l) % 2]);
}
}
sDp = newDp;
}
best = sDp[(i + j) % 2];
dp[x][i][j] = best + j;
}
}
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr);
ll n; cin >> n; g.resize(n + 1), state.resize(n + 1);
F0R(i, n - 1) {
ll a, b; cin >> a >> b;
g[a].push_back(b);
g[b].push_back(a);
}
F0R(i, n)
cin >> state[i + 1];
dp.resize(n + 1, vvi(2, vi(2, INF)));
solve(1, -1);
ll r = min(dp[1][state[1]][0], dp[1][state[1]][1]);
if (r >= INF) {
cout << "impossible\n";
} else
cout << r;
return 0;
}
