#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
#define x first
#define y second
ll cad[100005];
vector<ll> G[100005];
vector<vector<pair<ll,ll>>> cache;
pair<ll,ll> sol(ll node, ll padre, ll v)
{
if(cache[node][v].x!=-1) return cache[node][v];
pair<ll,ll> hoff={0,0}, hon={0,0};
ll moff=1e9, mon=1e9;
ll pl=false;
for(auto i:G[node])
{
if(i==padre) continue;
pl=true;
pair<ll,ll> temp=sol(i,node, 0);
hoff.x+=min(temp.x,temp.y);
hoff.y+=(temp.y<temp.x);
moff=min(moff,max(temp.x,temp.y)-min(temp.x,temp.y));
temp=sol(i,node, 1);
hon.x+=min(temp.x,temp.y);
hon.y+=(temp.y<temp.x);
mon=min(mon,max(temp.x,temp.y)-min(temp.x,temp.y));
}
ll sol1=1e9, sol2=1e9;
if(cad[node]==v)
{
if(hoff.y%2!=0) hoff.x+=moff;
sol1=hoff.x;
if(hon.y%2!=1) hon.x+=mon;
sol2=hon.x+1;
}
else
{
if(hoff.y%2!=1) hoff.x+=moff;
sol1=hoff.x;
if(hon.y%2!=0) hon.x+=mon;
sol2=hon.x+1;
}
//cerr<<node<<v<<" "<<sol1<<" "<<sol2<<"\n";
return cache[node][v]={sol1,sol2};
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);cout.tie(0);
ll n;
cin>>n;
cache.assign(n+4,vector<pair<ll,ll>>(3,{-1,0}));
for(int i=0; i<n-1; i++)
{
ll a,b;
cin>>a>>b;
G[a].push_back(b);
G[b].push_back(a);
}
for(int i=1; i<=n; i++)
{
cin>>cad[i];
}
ll t1=min(sol(1,0,0).x,sol(1,0,0).y);
if(t1>=1e9)
cout<<"impossible";
else
cout<<t1;
return 0;
}
