/*
//\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\\
\\ //
// 271828___182845__904523__53602__ \\
\\ 87___47____13______52____66__24_ //
// 97___75____72______47____09___36 \\
\\ 999595_____74______96____69___67 //
// 62___77____24______07____66__30_ \\
\\ 35___35____47______59____45713__ //
// \\
\\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\//
*/
#include <iostream>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_map>
#include <cmath>
#include <climits>
#include <algorithm>
#include <random>
#include <queue>
#include <deque>
#include <iomanip>
#include <string>
#include <tuple>
#include <bitset>
#include <chrono>
#include <ctime>
#include <fstream>
#include <stack>
#include <cstdio>
using namespace std;
using ll = long long;
const int N = 1e5 + 5;
const ll mod = 1e9 + 7, inf = 1e12;
int n;
ll dp[N][2][2];
vector<vector<int>> gp;
vector<int> col;
void dfs(int u, int p) {
for (int& v : gp[u]) {
if (v != p) dfs(v, u);
}
int cnt_child = gp[u].size() - (u != 0);
if (!cnt_child) {
if (col[u]) {
dp[u][0][0] = 0;
dp[u][1][1] = 1;
dp[u][0][1] = dp[u][1][0] = inf;
}
else {
dp[u][0][0] = dp[u][1][1] = inf;
dp[u][0][1] = 0;
dp[u][1][0] = 1;
}
return;
}
// dp[u][i][j]
// i -> gagat, j -> cnox
ll res = 0, mn = 2e9, cnt = 0;
for (int v : gp[u]) {
if (v == p) continue;
res += min(dp[v][0][0], dp[v][1][0]);
if (dp[v][1][0] <= dp[v][0][0]) {
++cnt;
}
mn = min(mn, abs(dp[v][0][0] - dp[v][1][0]));
}
if (col[u] && cnt % 2) dp[u][0][0] = res + mn;
else if (!col[u] && cnt % 2 == 0) dp[u][0][0] = res + mn;
else dp[u][0][0] = res;
if (!col[u] && cnt % 2) dp[u][0][1] = res + mn;
else if (col[u] && cnt % 2 == 0) dp[u][0][1] = res + mn;
else dp[u][0][1] = res;
res = 0, mn = 2e9, cnt = 0;
for (int& v : gp[u]) {
if (v == p) continue;
res += min(dp[v][0][1], dp[v][1][1]);
if (dp[v][1][1] <= dp[v][0][1]) ++cnt;
mn = min(mn, abs(dp[v][0][1] - dp[v][1][1]));
}
if (col[u] && cnt % 2 == 0) dp[u][1][0] = res + mn + 1;
else if (!col[u] && cnt % 2) dp[u][1][0] = res + mn + 1;
else dp[u][1][0] = res + 1;
if (!col[u] && cnt % 2 == 0) dp[u][1][1] = res + mn + 1;
else if (col[u] && cnt % 2) dp[u][1][1] = res + mn + 1;
else dp[u][1][1] = res + 1;
//if(!u)
}
void solve() {
cin >> n;
gp = vector<vector<int>>(n);
col = vector<int>(n);
for (int i = 1; i < n; ++i) {
int u, v; cin >> u >> v;
gp[--u].push_back(--v);
gp[v].push_back(u);
}
for (int i = 0; i < n; ++i) cin >> col[i], col[i] = !col[i];
dfs(0, -1);
/*for (int i = 0; i < n; ++i) {
cout << dp[i][0][0] << " " << dp[i][0][1] << " " << dp[i][1][0] << " " << dp[i][1][1] << "\n";
}*/
ll res = min(dp[0][0][0], dp[0][1][0]);
if (res > n) cout << "impossible\n";
else cout << res << "\n";
}
int main() {
ios_base::sync_with_stdio(0);
cin.tie(NULL);
//int ; cin >> _; while (--)
solve();
}
