#include <cstdio>
#include <cstdlib>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstring>
#include <iostream>
#include <iomanip>
#include <limits.h>
#include <set>
#include <string>
#include <queue>
#include <stack>
#include <unordered_map>
#include <unordered_set>
#include <deque>
#include <map>
#include <chrono>
#include <random>
#include <bitset>
#include <tuple>
#include <cassert>
#define SZ(x) (LL)(x.size())
#define FR(i,a,b) for(LL i=(a);i<(b);++i)
#define FOR(i,n) FR(i,0,n)
#define FAST ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)
#define A first
#define B second
#define mp(a,b) make_pair(a,b)
typedef long long LL;
typedef long double LD;
typedef unsigned long long ULL;
typedef unsigned __int128 U128;
typedef __int128 I128;
typedef std::pair<int,int> PII;
typedef std::pair<LL,LL> PLL;
typedef std::pair<LD,LD> PLD;
using namespace std;
const LL MAXN=1e5+5;
vector<LL> adj[MAXN];
LL dp[MAXN][4];
LL b[MAXN];
void dfs(LL u, LL par){
LL sum0=0, sum1=0;
for (LL v:adj[u]){
if (v==par) continue;
dfs(v,u);
sum0+=dp[v][0];
sum1+=dp[v][1];
}
//case 1: dont toggle u
vector<LL> tmp;
for (LL v:adj[u]) tmp.push_back(dp[v][2]-dp[v][0]);
sort(tmp.begin(),tmp.end());
LL cur=0;
dp[u][b[u]]=sum0;
FOR(i,SZ(tmp)){
if (tmp[i]>=0) break;
cur+=tmp[i];
if ((i+1)%2==b[u]) dp[u][0]=sum0+cur; //weve taken the same parity of b[u], bulb is now off
else dp[u][1]=sum0+cur;
}
//case 2: toggle u
tmp.clear();
for (LL v:adj[u]) tmp.push_back(dp[v][3]-dp[v][1]);
sort(tmp.begin(),tmp.end());
sum1++;
cur=0;
dp[u][(b[u]+1)%2+2]=sum1;
FOR(i,SZ(tmp)){
if (tmp[i]>=0) break;
cur+=tmp[i];
if (i%2==b[u]) dp[u][2]=sum1+cur; //weve taken opp parity of b[u], bulb is now off
else dp[u][3]=sum1+cur;
}
}
signed main(){
FAST;
LL n; cin>>n;
FOR(i,n-1){
LL u,v; cin>>u>>v;
adj[u].push_back(v);
adj[v].push_back(u);
}
FR(i,1,n+1) cin>>b[i];
FR(i,1,n+1) FOR(j,4) dp[i][j]=1e18;
dfs(1,-1);
/*FR(i,1,n+1){
FOR(j,4) cout<<dp[i][j]<<" ";
cout<<"\n";
}*/
LL ans=min(dp[1][0], dp[1][2]);
if (ans>=1e18) cout<<"impossible";
else cout<<ans;
return 0;
}
