#pragma GCC optimize("Ofast,unroll-loops")
#include<bits/stdc++.h>
#define int long long
//#define ll long long
#define down cout<<'\n';
#define debug cout<<" cucuucucuuu",down
#define NHP ios_base::sync_with_stdio(0);cout.tie(0);cin.tie(0);
#define modwwe int t;cin>>t; while(t--)
#define bit(i,j) (i>>j&1)
#define sobit(a) __builtin_popcountll(a)
#define task "test"
#define fin(x) freopen(x".inp","r",stdin)
#define fou(x) freopen(x".out","w",stdout)
#define pb push_back
#define checktime cerr << (double)clock() / CLOCKS_PER_SEC * 1000 << " ms";
using namespace std;
#define getchar_unlocked getchar
inline int scan()
{
char c = getchar_unlocked();
int x = 0;
while(c<'0'||c>'9')
{
c=getchar_unlocked();
}
while(c>='0'&&c<='9')
{
x=(x<<1)+(x<<3)+c-'0';
c=getchar_unlocked();
}
return x;
}
void phongbeo();
const int inf=1e9;
const int mod2=998244353;
const int mod1=998244353;
struct icd
{
long double a;
int b;
};
struct ib
{
int a;
int b;
};
struct ic
{
int a,b,c;
};
struct id
{
int a,b,c,d;
};
struct ie
{
int a,b,c,d,e;
};
int n,m,s1,s2,s4,s3,sf,k,s5,s6,mx,s7,s8,s9,mx2,res,dem2=0,dem=0,s33,dem3,dem4,l,r,mid,l2,r2,center;
int i,s10,s12;
int kk;
int el=29;
main()
{
#ifndef ONLINE_JUDGE
/// fin(task),fou(task);
#endif
NHP
/// cin>>s1;
///modwwe
phongbeo(),down
//checktime
}
int dp[100001][2][2];
bool b[100001];
vector<int> v[100001];
void dfs(int x,int y)
{
/// 0 0 no tat cac con no tat
/// 0 1 no tat cac con no bat
/// 1 0 no bat cac con no tat
/// 1 1 no va cac con bat
dp[x][b[x]][1]=0;
dp[x][1-b[x]][1]=1e12;
dp[x][1-b[x]][0]=1e12;
dp[x][b[x]][0]=0;
for(auto f:v[x])
{
if(f!=y)
{
dfs(f,x);
s2=dp[x][0][0];
s3=dp[x][0][1];
s4=dp[x][1][0];
s5=dp[x][1][1];
dp[x][0][0]=min({dp[x][0][0]+dp[f][0][0],s4+dp[f][1][1]+1});
dp[x][1][0]=min({dp[x][1][0]+dp[f][0][0],s2+dp[f][1][1]+1});
dp[x][0][1]=min({dp[x][0][1]+dp[f][1][0],s5+dp[f][0][1]+1});
dp[x][1][1]=min({dp[x][1][1]+dp[f][1][0],s3+dp[f][0][1]+1});
}
}
if(y==0)
dp[1][0][0]=min(dp[1][0][0],dp[1][1][1]+1);
}
void phongbeo()
{
cin>>n;
for(int i=1; i<n; i++)
cin>>l>>r,v[l].pb(r),v[r].pb(l);
for(int i=1; i<=n; i++)
cin>>b[i];
dfs(1,0);
if(dp[1][0][0]<=1e12)
cout<<dp[1][0][0];
else cout<<"impossible";
}
