#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define dbg(x) cerr << #x << " " << x << "\n"
const int MAX_N = 1e5;
vector <int> gr[1 + MAX_N];
int dp[1 + MAX_N][2][2];
int on[1 + MAX_N];
/// dp[node][switched 0/1][value given 0/1] =
void calc_dp (int node, int par) {
int offset1 = 0, offset2 = 0;
int flip_offset1 = (1 << 30), flip_offset2 = (1 << 30);
bool state1 = not on[node], state2 = on[node];
for (int son : gr[node]) {
if (son != par) {
calc_dp (son, node);
/// we switch node
if (dp[son][0][1] < dp[son][1][1]) {
offset1 += dp[son][0][1];
flip_offset1 = min (flip_offset1, dp[son][1][1] - dp[son][0][1]);
}
else {
offset1 += dp[son][1][1];
flip_offset1 = min (flip_offset1, dp[son][0][1] - dp[son][1][1]);
state1 ^= 1;
}
/// we don't switch node
if (dp[son][0][0] < dp[son][1][0]) {
offset2 += dp[son][0][0];
flip_offset2 = min (flip_offset2, dp[son][1][0] - dp[son][0][0]);
}
else {
offset2 += dp[son][1][0];
flip_offset2 = min (flip_offset2, dp[son][0][0] - dp[son][1][0]);
state2 ^= 1;
}
}
}
dp[node][1][state1] = offset1 + 1;
dp[node][1][not state1] = offset1 + flip_offset1;
dp[node][0][state2] = offset2;
dp[node][0][not state2] = offset2 + flip_offset2;
}
int main () {
ios::sync_with_stdio (false);
cin.tie (0); cout.tie (0);
int n;
cin >> n;
for (int i = 1; i < n; i++) {
int x, y;
cin >> x >> y;
gr[x].push_back (y);
gr[y].push_back (x);
}
for (int i = 1; i <= n; i++)
cin >> on[i];
calc_dp (1, 0);
int ans = min (dp[1][0][0], dp[1][1][0]);
if (ans == (1 << 30))
cout << "impossible\n";
else
cout << ans << "\n";
return 0;
}
