#include "bits/stdc++.h"
using namespace std;
const int MAXN = 100005;
const int INF = 2147483647;
int yes[MAXN],no[MAXN],diffyes[MAXN],diffno[MAXN];
vector<int> graph[MAXN];
bool states[MAXN];
int cur,total,mindiff; bool possible;
void dfs(int u, int v) {
for (int i : graph[u]) if (i != v) dfs(i,u);
cur = 0, total = 0, mindiff = INF; possible = true;
for (int i : graph[u]) {
if (i == v) continue;
if (yes[i]<no[i]) cur++;
if (min(yes[i],no[i]) == INF) { possible = false; break; }
total+=min(yes[i],no[i]);
if (max(yes[i],no[i]) != INF) mindiff = min(mindiff,abs(yes[i]-no[i]));
}
if (possible) {
if (cur%2 == states[u]) {
no[u] = total;
if (mindiff!=INF) diffno[u] = total+mindiff;
} else {
diffno[u] = total;
if (mindiff!=INF) no[u] = total+mindiff;
}
}
cur = 1, total = 1, mindiff = INF; possible = true;
for (int i : graph[u]) {
if (i == v) continue;
if (diffyes[i]<diffno[i]) cur++;
if (min(diffyes[i],diffno[i]) == INF) { possible = false; break; }
total+=min(diffyes[i],diffno[i]);
if (max(diffyes[i],diffno[i]) != INF) mindiff = min(mindiff,abs(diffyes[i]-diffno[i]));
}
if (possible) {
if (cur%2 == states[u]) {
yes[u] = total;
if (mindiff!=INF) diffyes[u] = total+mindiff;
} else {
diffyes[u] = total;
if (mindiff!=INF) yes[u] = total+mindiff;
}
}
}
int main() {
#ifdef LOCAL
freopen("submission/input.in", "r", stdin);
freopen("submission/output.out", "w", stdout);
#else
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#endif
int n; cin >> n;
for (int i = 1; i<n; i++) { int x,y; cin >> x >> y; graph[x].push_back(y); graph[y].push_back(x); }
for (int i = 1; i<=n; i++) cin >> states[i];
fill(yes,yes+MAXN,INF); fill(no,no+MAXN,INF); fill(diffyes,diffyes+MAXN,INF); fill(diffno,diffno+MAXN,INF);
dfs(1,0);
int x = min(yes[1],no[1]);
if (x == INF) cout << "impossible\n";
else cout << x << '\n';
return 0;
}
