#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define mp make_pair
#define pii pair<int,int>
#define ss second
#define ff first
#define piii pair<int,pii>
#define debu(x) (cerr << #x << " = "<< x << "\n")
#define debu2(x,y) (cerr << #x << " = "<< x << " " << #y << " = " << y << "\n")
#define debu3(x,y,z) (cerr << #x << " = "<< x << " " << #y << " = " << y << " " << #z << " = " << z<< "\n")
#define debu4(x,y,z,a) (cerr << #x << " = "<< x << " " << #y << " = " << y << " " << #z << " = " << z<< " "<<a<<"\n")
#define bitout(x,y) {\
cerr << #x << " : ";\
for (int justforbits = y; justforbits >=0; justforbits--)cout << (((1 << justforbits) & x)>=1);\
cout << "\n";\
}
#define rangeout(j,rangestart,rangeend) {\
cerr << "outputting" << #j<< ":\n";\
for (int forrang = rangestart; forrang <= rangeend; forrang++)cerr << j[forrang] << " ";\
cerr<<"\n";\
}
#define c1 {cerr << "Checkpoint 1! \n\n";cerr.flush();}
#define c2 {cerr << "Checkpoint 2! \n\n";cerr.flush();}
#define c3 {cerr << "Checkpoint 3! \n\n";cerr.flush();}
#define c4 {cerr << "Checkpoint 4! \n\n";cerr.flush();}
#define defN 100001
vector<vector<int>>adjlist(defN);
vector<vector<int>>childlist(defN);
vector<vector<int>>depths;
vector<bool>visited(defN,false);
void dfs(int cn,int cd)
{
visited[cn]=true;
if(depths.size()==cd){depths.pb(vector<int>());}
depths[cd].pb(cn);
for(int to:adjlist[cn])
{
if(visited[to])continue;
childlist[cn].pb(to);
dfs(to,cd+1);
}
}
signed main()
{
int t1,t2,t3,t4;
mt19937_64 rnd(chrono::high_resolution_clock::now().time_since_epoch().count());
//ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
int n;cin>>n;
vector<vector<vector<int>>>dp(n+1,vector<vector<int>>(2,vector<int>(2,INT_MAX)));
//are you black or white
//did you turn or not
for(int a=1;a<n;a++)
{
cin>>t1>>t2;
adjlist[t1].pb(t2);
adjlist[t2].pb(t1);
}
dfs(1,0);
int maxdepth=depths.size()-1;
vector<bool>onn(n+1);
for(int i=1;i<=n;i++)
{
cin>>t1;
onn[i]=t1;
}
for(int i=maxdepth;i>=0;i--)
{
for(int node:depths[i])
{
if(childlist[node].size()==0)
{
//debu2(childlist[node].size(),node);
dp[node][onn[node]][0]=0;
dp[node][!onn[node]][1]=1;
}
else
{
//do for all whites +flip and no flip
//do for all blacks +flip and no flip
//you do a thereafter flip if its the second run
bool firsttype=0;
for(int ii=0;ii<2;ii++)
{
int currans=0;
priority_queue<int,vector<int>,greater<int>>pq;
for(int x:childlist[node])
{
currans+=dp[x][firsttype][0];
pq.push(dp[x][firsttype][1]-dp[x][firsttype][0]);
}
bool now=firsttype?1-onn[node]:onn[node];
dp[node][now][firsttype]=currans+firsttype;
//debu4(node,now,firsttype,currans+firsttype);
while(!pq.empty())
{
now^=1;
currans+=(pq.top());
pq.pop();
//debu4(node,now,firsttype,currans+firsttype);
dp[node][now][firsttype]=min(dp[node][now][firsttype],currans+firsttype);
}
firsttype^=1;
}
}
}
}
int ans=min(dp[1][0][1],dp[1][0][0]);
if(ans>=INT_MAX)cout<<"impossible";
else cout<<ans;
}
