#include <bits/stdc++.h>
using namespace std;
template<typename T>
void out(T x) { cout << x << endl; exit(0); }
#define watch(x) cout << (#x) << " is " << (x) << endl
const int maxn = 1e5 + 5;
const int inf = 1e7;
void setmin(int &x, int y) {
x = min(x, y);
}
int n;
vector<int> g[maxn];
int a[maxn];
int dp[maxn][2][2];
//dp[at][value][flip/noflip] = min flips
void dfs(int at, int p) {
for (int to: g[at]) {
if (to == p) continue;
dfs(to, at);
}
for (int flip: {0,1}) {
dp[at][a[at]^flip][flip] = flip;
dp[at][a[at]^flip^1][flip] = inf;
for (int to: g[at]) {
if (to == p) continue;
vector<int> ndp = {inf, inf};
for (int tval: {0,1}) {
if ((tval^flip) == 0) {
for (int tflip: {0,1}) {
for (int aval: {0,1}) {
setmin(ndp[aval^tflip], dp[at][aval][flip]+dp[to][tval][tflip]);
}
}
}
}
dp[at][0][flip] = ndp[0];
dp[at][1][flip] = ndp[1];
}
}
}
int main() {
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin>>n;
for (int i=0; i<n-1; i++) {
int u,v; cin>>u>>v;
--u; --v;
g[u].push_back(v);
g[v].push_back(u);
}
for (int i=0; i<n; i++) {
cin>>a[i];
}
dfs(0, -1);
// for (int v: {0,1}) {
// for (int f: {0,1}) {
// cout<<v<<" "<<f<<": "<<dp[0][v][f]<<endl;
// }
// }
int ans = inf;
for (int flip: {0,1}) {
ans = min(ans, dp[0][0][flip]);
}
if (ans == inf) out("impossible");
out(ans);
return 0;
}
