#include "bits/extc++.h"
using namespace std;
template <typename T>
void dbgh(const T& t) {
cerr << t << endl;
}
template <typename T, typename... U>
void dbgh(const T& t, const U&... u) {
cerr << t << " | ";
dbgh(u...);
}
#ifdef DEBUG
#define dbg(...) \
cerr << "L" << __LINE__ << " [" << #__VA_ARGS__ << "]" \
<< ": "; \
dbgh(__VA_ARGS__)
#else
#define cerr \
if (false) \
cerr
#define dbg(...)
#endif
#define endl "\n"
#define long int64_t
#define sz(x) int((x).size())
const int maxn = 1e5 + 5;
bool arr[maxn];
int memo[maxn][2][2];
vector<int> graph[maxn];
void pdfs(int u) {
for (auto& v : graph[u]) {
graph[v].erase(find(begin(graph[v]), end(graph[v]), u));
pdfs(v);
}
}
int dp(int u, bool p, bool c) {
int &ans = memo[u][p][c];
if (ans != -1) {
return ans;
}
int cdp[2];
cdp[0] = cdp[1] = 1e9;
cdp[p ^ c] = 0;
for (auto& v : graph[u]) {
int ndp[2];
for (int i = 0; i < 2; i++) {
ndp[i] = min({int(1e9), cdp[i] + dp(v, c, false), cdp[i ^ 1] + 1 + dp(v, c, true)});
}
cdp[0] = ndp[0];
cdp[1] = ndp[1];
}
return ans = cdp[arr[u]];
}
void solve() {
int n;
cin >> n;
for (int i = 0; i < n - 1; i++) {
int u, v;
cin >> u >> v;
u--;
v--;
graph[u].push_back(v);
graph[v].push_back(u);
}
for (int i = 0; i < n; i++) {
cin >> arr[i];
}
pdfs(0);
memset(memo, -1, sizeof(memo));
int ans = min(dp(0, false, false), 1 + dp(0, false, true));
if (ans < int(1e9)) {
cout << ans << endl;
} else {
cout << "impossible" << endl;
}
}
int main() {
cin.tie(nullptr);
ios_base::sync_with_stdio(false);
cin.exceptions(ios::failbit);
solve();
}
