#pragma GCC optimize("O3,inline")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
using namespace __gnu_pbds;
using namespace std;
using ll = long long;
#define int ll
using ii = pair<int, int>;
using iii = pair<ii, int>;
constexpr int INF = 1e18 + 5;
constexpr int MAXN = 100000 + 5;
constexpr int MOD = 1e9 + 7;
int dp[MAXN][2][2];
vector<int> adjlist[MAXN];
bool state[MAXN];
void dfs(int x, int pa) {
dp[x][0][!state[x]] = 0;
dp[x][0][state[x]] = INF;
dp[x][1][!state[x]] = INF;
dp[x][1][state[x]] = 1;
for (int ch : adjlist[x]) {
if (ch == pa) continue;
dfs(ch, x);
int temp[2][2];
temp[0][0] = min(
dp[x][0][0] + dp[ch][0][1],
dp[x][0][1] + dp[ch][1][1]
);
temp[0][1] = min(
dp[x][0][0] + dp[ch][1][1],
dp[x][0][1] + dp[ch][0][1]
);
temp[1][0] = min(
dp[x][1][0] + dp[ch][0][0],
dp[x][1][1] + dp[ch][1][0]
);
temp[1][1] = min(
dp[x][1][0] + dp[ch][1][0],
dp[x][1][1] + dp[ch][0][0]
);
dp[x][0][0] = temp[0][0];
dp[x][1][0] = temp[1][0];
dp[x][0][1] = temp[0][1];
dp[x][1][1] = temp[1][1];
}
}
signed main() {
ios_base::sync_with_stdio(false); cin.tie(nullptr); cout.tie(nullptr);
int N; cin >> N;
for (int i = 1; i < N; ++i) {
int u, v; cin >> u >> v;
adjlist[u].emplace_back(v);
adjlist[v].emplace_back(u);
}
for (int i = 1; i <= N; ++i) cin >> state[i];
dfs(1, -1);
int ans = min(dp[1][0][1], dp[1][1][1]);
cout << (ans >= INF ? "impossible" : to_string(ans));
}
