#include <iostream>
#include <vector>
#include <set>
#include <queue>
#include <deque>
#include <map>
#include <stack>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <climits>
using namespace std;
using ll = long long;
int n;
vector<int> clr;
vector<vector<int>> gp;
vector<vector<vector<int>>> dp;
void DPcreator(int u, int p, int nerk)
{
multiset<pair<int, int>> st, zibil;
bool impossible = 0;
int MinChange = 2e9;
for (int& v : gp[u])
{
if (v == p) continue;
if (dp[v][0][nerk] < 2e9 && dp[v][1][nerk] < 2e9)
{
st.insert({ min(dp[v][0][nerk], dp[v][1][nerk]), (dp[v][0][nerk] < dp[v][1][nerk] ? 0 : 1) });
MinChange = min(MinChange, abs(dp[v][0][nerk] - dp[v][1][nerk]));
}
else if (dp[v][0][nerk] < 2e9)
st.insert({ dp[v][0][nerk], 0 });
else if (dp[v][1][nerk] < 2e9)
st.insert({ dp[v][1][nerk], 1 });
else impossible = 1;
}
int sum = 0, cnt = clr[u] + nerk;
for (pair<int, int> i : st)
{
cnt += i.second;
sum += i.first;
}
int NotChanged = sum + nerk;
sum += MinChange;
int Changed = sum + nerk;
//
if (impossible) NotChanged = Changed = 2e9;
//
if (cnt & 1) dp[u][nerk][0] = Changed, dp[u][nerk][1] = NotChanged;
else dp[u][nerk][0] = NotChanged, dp[u][nerk][1] = Changed;
}
void dfs(int u, int p)
{
if (gp[u].size() == 1 && u != 0)
{
if (clr[u])
{
dp[u][0][0] = 2e9;
dp[u][0][1] = 0;
dp[u][1][0] = 1;
dp[u][1][1] = 2e9;
}
else
{
dp[u][0][0] = 0;
dp[u][0][1] = 2e9;
dp[u][1][0] = 2e9;
dp[u][1][1] = 1;
}
return;
}
for (int& v : gp[u])
{
if (v == p) continue;
if (dp[v][0][0] == -1) dfs(v, u);
}
DPcreator(u, p, 0);
DPcreator(u, p, 1);
}
int main() {
int n; cin >> n;
gp = vector<vector<int>>(n);
dp = vector<vector<vector<int>>>(n, { {-1, -1}, {-1, -1} });
for (int i = 1; i < n; i++)
{
int u, v; cin >> u >> v;
gp[--u].push_back(--v);
gp[v].push_back(u);
}
clr = vector<int>(n);
for (int& i : clr) cin >> i;
dfs(0, -1);
int ans = min(dp[0][0][0], dp[0][1][0]);
if (ans >= 2e9) cout << "impossible\n";
else cout << ans << endl;
//for (int i = 0; i < n; i++) { for (int j = 0; j < 2; j++) for (int k = 0; k < 2; k++) printf("dp[%d][%d][%d] = %d\n", i + 1, j, k, dp[i][j][k]); puts(""); }
return 0;
}
