#include <bits/stdc++.h>
#define Loop(x,l,r) for (ll x = (l); x < (ll)(r); ++x)
#define LoopR(x,l,r) for (ll x = (r)-1; x >= (ll)(l); --x)
typedef long long ll;
typedef std::pair<int, int> pii;
typedef std::pair<ll , ll > pll;
using namespace std;
const int N = 100'010;
vector<int> A[N];
int a[N];
int n;
array<int, 4> dfs(int v, int p)
{
int mnch[2] = {N, N};
int sum[2] = {0, 1};
int s[2] = {a[v], !a[v]};
for (int u : A[v]) {
if (u == p)
continue;
auto arr = dfs(u, v);
if (arr[0] < arr[2]) {
sum[0] += arr[0];
mnch[0] = min(mnch[0], arr[2]-arr[0]);
} else {
sum[0] += arr[2];
mnch[0] = min(mnch[0], arr[0]-arr[2]);
s[0] ^= 1;
}
if (arr[1] < arr[3]) {
sum[1] += arr[1];
mnch[1] = min(mnch[1], arr[3]-arr[1]);
} else {
sum[1] += arr[3];
mnch[1] = min(mnch[1], arr[1]-arr[3]);
s[1] ^= 1;
}
}
array<int, 4> ans;
ans[0] = s[0]? sum[0] + mnch[0]: sum[0];
ans[1] = s[0]? sum[0]: sum[0] + mnch[0];
ans[2] = s[1]? sum[1] + mnch[1]: sum[1];
ans[3] = s[1]? sum[1]: sum[1] + mnch[1];
return ans;
}
int main()
{
cin.tie(0) -> sync_with_stdio(false);
cin >> n;
Loop (i,1,n) {
int v, u;
cin >> v >> u;
--v; --u;
A[v].push_back(u);
A[u].push_back(v);
}
Loop (i,0,n)
cin >> a[i];
auto arr = dfs(0, -1);
int ans = min(arr[0], arr[2]);
if (ans >= N)
cout << "impossible\n";
else
cout << ans << '\n';
}
