#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int N=1e5+5;
int n, r[N];
ll a[N], b[N], c[N], d[N];
vector<int>g[N];
void dfs(const int &v, const int &p=-1){
b[v]=d[v]=1;
vector<int>x, y;
for(int u: g[v]){
if(u!=p){
dfs(u, v);
a[v]+=a[u];
c[v]+=a[u];
x.push_back(b[u]-a[u]);
b[v]+=c[u];
d[v]+=c[u];
y.push_back(d[u]-c[u]);
}
}
sort(x.begin(), x.end());
sort(y.begin(), y.end());
ll aa=1e18, bb=1e18, cc=1e18, dd=1e18;
if(r[v]){
aa=0;
dd=0;
}
else{
bb=0;
cc=0;
}
ll sm=0;
bool h=r[v];
for(int u: x){
sm+=u;
if(h)
cc=min(cc, sm);
else
aa=min(aa, sm);
h=!h;
}
h=r[v];
sm=0;
for(int u: y){
sm+=u;
if(h)
bb=min(bb, sm);
else
dd=min(dd, sm);
h=!h;
}
a[v]+=aa;
b[v]+=bb;
c[v]+=cc;
d[v]+=dd;
ll zz=1e9;
a[v]=min(a[v], zz);
b[v]=min(b[v], zz);
c[v]=min(c[v], zz);
d[v]=min(d[v], zz);
}
int main(){
cin>>n;
for(int i=1;i<n;++i){
int u, v;
cin>>u>>v;
g[--u].push_back(--v);
g[v].push_back(u);
}
for(int i=0;i<n;++i){
cin>>r[i];
r[i]=1-r[i];
}
dfs(0);
int z=min(a[0], b[0]);
if(z>n)
cout<<"impossible\n";
else
cout<<z<<'\n';
}
