#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int MX = 1e5 + 5;
int N;
int c[MX];
vector<int> g[MX];
ll dp[MX][2][2];
void dfs(int v, int p) {
if(g[v].size() == 1 && v != 1) {
dp[v][c[v]][0] = 0;
dp[v][c[v] ^ 1][1] = 1;
return;
}
for(auto u : g[v]) {
if(u == p) continue;
dfs(u, v);
}
ll sum = 0, cnt = 0, mn = 1e18;
for(auto u : g[v]) {
if(u == p) continue;
if(dp[u][0][0] < dp[u][0][1]) {
sum += dp[u][0][0];
mn = min(mn, dp[u][0][1] - dp[u][0][0]);
} else {
sum += dp[u][0][1];
mn = min(mn, dp[u][0][0] - dp[u][0][1]);
cnt++;
}
}
cnt &= 1;
dp[v][cnt ^ c[v]][0] = min(dp[v][cnt ^ c[v]][0], sum);
cnt ^= 1;
dp[v][cnt ^ c[v]][0] = min(dp[v][cnt ^ c[v]][0], sum + mn);
sum = 0, cnt = 0, mn = 1e18;
for(auto u : g[v]) {
if(u == p) continue;
if(dp[u][1][0] < dp[u][1][1]) {
sum += dp[u][1][0];
mn = min(mn, dp[u][1][1] - dp[u][1][0]);
} else {
sum += dp[u][1][1];
mn = min(mn, dp[u][1][0] - dp[u][1][1]);
cnt++;
}
}
cnt &= 1;
cnt ^= 1;
dp[v][cnt ^ c[v]][1] = min(dp[v][cnt ^ c[v]][1], sum + 1);
cnt ^= 1;
dp[v][cnt ^ c[v]][1] = min(dp[v][cnt ^ c[v]][1], sum + mn + 1);
}
int main() {
cin.tie(0); ios_base::sync_with_stdio(0);
for(int i = 0; i < MX; i++)
for(int j = 0; j < 2; j++)
for(int k = 0; k < 2; k++)
dp[i][j][k] = 1e18;
cin >> N;
for(int i = 0; i < N - 1; i++) {
int u, v;
cin >> u >> v;
g[u].push_back(v);
g[v].push_back(u);
}
for(int i = 1; i <= N; i++) cin >> c[i];
dfs(1, 0);
ll res = min(dp[1][0][0], dp[1][0][1]);
if(res == 1e18) {
cout << "impossible\n";
} else {
cout << res << '\n';
}
}
