#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define pii pair<int, int>
#define f first
#define s second
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) (int)((x).size())
#define pb push_back
#define MX(a,b) a = max(a,b)
#define MN(a,b) a = min(a,b)
#define REP(i,n) for (int i = 0; i<n; ++i)
#define REP1(i,n) for (int i = 1; i<=n; ++i)
#ifdef BALBIT
#define bug(...) cerr<<"#"<<__LINE__<<": "<<#__VA_ARGS__<<"- ", _do(__VA_ARGS__)
template<typename T> void _do( T && x) {cerr<<x<<endl;}
template<typename T, typename ...S> void _do( T && x, S && ...y) {cerr<<x<<", "; _do(y...);}
#else
#define bug(...)
#define endl '\n'
#endif // BALBIT
const int maxn = 1e5+6;
int dp[maxn][2][2]; // v, me, par
vector<int >g[maxn];
bool lit[maxn];
void dfs(int v, int p) {
for (int u : g[v]) {
if (u != p) {
dfs(u,v);
}
}
REP(tk, 2) {
int me = lit[v] ^ tk;
int sig0 = 0;
vector<int> go;
for (int u : g[v]) {
if (u != p) {
sig0 += dp[u][tk][0];
go.pb(dp[u][tk][1] - dp[u][tk][0]);
}
}
sort(ALL(go));
int sm = 0;
REP(t, SZ(go)+1) {
MN(dp[v][me^(t%2)][tk], sig0 + sm + tk);
if (t < SZ(go)) sm += go[t];
}
bug(v, tk, dp[v][0][tk], dp[v][1][tk]);
}
}
signed main(){
ios::sync_with_stdio(0), cin.tie(0);
memset(dp, 0x3f, sizeof dp);
int n; cin>>n;
REP(i,n-1) {
int a,b; cin>>a>>b; g[a].pb(b); g[b].pb(a);
}
REP1(i,n) {
cin>>lit[i];
}
dfs(1, -1);
int re = min(dp[1][0][1], dp[1][0][0]);
if (re > n) cout<<"impossible"<<endl;
else cout<<re<<endl;
}
