//by szh
#include<bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pii pair<int,int>
#define pll pair<long long,long long>
#define pb push_back
#define debug(x) cerr<<#x<<"="<<x<<endl
#define pq priority_queue
#define inf 0x3f
#define rep(i,a,b) for (int i=a;i<(b);i++)
#define MP make_pair
#define SZ(x) (int(x.size()))
#define ll long long
#define mod 1000000007
#define ALL(x) x.begin(),x.end()
void inc(int &a,int b) {a=(a+b)%mod;}
void dec(int &a,int b) {a=(a-b+mod)%mod;}
int lowbit(int x) {return x&(-x);}
ll p0w(ll base,ll p) {ll ret=1;while(p>0){if (p%2ll==1ll) ret=ret*base%mod;base=base*base%mod;p/=2ll;}return ret;}
const int maxn = 1e5+10;
int n;
vector <int> edge[maxn];
int on[maxn];
int f[maxn][2][2];
void dfs(int u,int fa) {
int tmp[2][2][2]; //(i,j,k) j:press?, k:on/off
int cur=0,lst=1;
tmp[lst][0][0] = tmp[lst][0][1] = 0;
tmp[lst][1][0] = tmp[lst][1][1] = maxn;
for (int v:edge[u]) {
if (v==fa) continue;
dfs(v,u);
rep(i,0,2) {
tmp[cur][0][i] = min(tmp[lst][0][i]+f[v][0][i],tmp[lst][1][i]+f[v][1][i]);
tmp[cur][1][i] = min(tmp[lst][0][i]+f[v][1][i],tmp[lst][1][i]+f[v][0][i]);
}
swap(cur,lst);
}
//press u
f[u][1][on[u]] = tmp[lst][1][1] + 1;
f[u][1][on[u]^1] = tmp[lst][0][1] + 1;
//does not press u
f[u][0][on[u]] = tmp[lst][0][0];
f[u][0][on[u]^1] = tmp[lst][1][0];
//rep(i,0,2) rep(j,0,2) cout<<u<<" "<<i<<" "<<j<<" "<<f[u][i][j]<<endl;
return;
}
int main() {
// freopen("input.txt","r",stdin);
std::ios::sync_with_stdio(false);cin.tie(0);
cin>>n;
rep(i,1,n) {
int u,v;
cin>>u>>v;
edge[u].pb(v);
edge[v].pb(u);
}
rep(i,1,n+1) cin>>on[i];
dfs(1,-1);
if (min(f[1][0][0],f[1][1][0])>n) cout<<"impossible";
else cout<<min(f[1][0][0],f[1][1][0]);
return 0;
}
