#define _CRT_SECURE_NO_WARNINGS
#include <bits/stdc++.h>
#define ll long long
#define lld long double
#define ff first
#define ss second
#define pb push_back
#define vr(v) v.begin(),v.end()
#define rv(v) v.rbegin(),v.rend()
#define Code ios_base::sync_with_stdio(false);
#define By cin.tie(NULL);
#define Davit cout.tie(NULL);
using namespace std;
vector<vector<int>> gp;
vector<int> col;
const int INF = 1e6;
ll dp[100005][2][2];
void dfs(int u, int p) {
for (auto x: gp[u]) {
if (x != p)dfs(x, u);
}
if ((int) gp[u].size() == 1 && u != 0) {//leaves
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] = INF;
dp[u][1][1] = INF;
dp[u][1][0] = 1;
dp[u][0][1] = 0;//we count it later (during merge)
}
return;
}
ll res = 0, cnt = 0, diff = INF;
//dp[x][0/1][0]
for (auto x: gp[u]) {
if (x == p)continue;
res += min(dp[x][0][0], dp[x][1][0]);
if (dp[x][1][0] < dp[x][0][0])cnt++;
diff = min(diff, abs(dp[x][0][0] - dp[x][1][0]));
}
dp[u][0][0] = res;
if (col[u] == cnt % 2)dp[u][0][0] += diff;
dp[u][0][1] = res;
if (col[u] != cnt % 2)dp[u][0][1] += diff;
res = 0, cnt = 0, diff = INF;
for (auto x: gp[u]) {
if (x == p)continue;
res += min(dp[x][0][1], dp[x][1][1]);
if (dp[x][1][1] < dp[x][0][1])cnt++;
diff = min(diff, abs(dp[x][0][1] - dp[x][1][1]));
}
res++;
dp[u][1][0] = res;
if (col[u] != cnt % 2)dp[u][1][0] += diff;
dp[u][1][1] = res;
if (col[u] == cnt % 2)dp[u][1][1] += diff;
}
int main() {
int n;
cin >> n;
gp.resize(n);
col.resize(n);
for (int i = 0; i < n - 1; i++) {
int u, v;
cin >> u >> v;
u--, v--;
gp[u].pb(v);
gp[v].pb(u);
}
for (int i = 0; i < n; i++)cin >> col[i], col[i] = !col[i];
dfs(0, -1);
ll res = min(dp[0][0][0], dp[0][1][0]);
if (res > n) cout << "impossible\n";
else cout << res << "\n";
}
