#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define st first
#define nd second
const ll maxn = (1e+5)+9;
ll dp[maxn][2][2];
vector <int> t[maxn];
bool stp[maxn];
void DFS(int v , int o)
{
ll maxdif = maxn+13;
ll aktM = 0 ,res = 0;
for(auto p : t[v])
{
if(p == o)continue;
DFS(p,v);
}
for(int j = 0 ;j < 2; j++)//dp[v][j]
{
//dp[v][j][0] - nie uruchamiam u siebie
maxdif =maxn+13 , aktM = 0;
res = 0;
for(auto p : t[v])
{
if(p == o)continue;
res += min(dp[p][0][0],dp[p][0][1]);
if(dp[p][0][1] < dp[p][0][0])
{
aktM ^= 1;
}
maxdif= min(maxdif , max(dp[p][0][0],dp[p][0][1])-min(dp[p][0][0],dp[p][0][1]));
}
if(((j^stp[v])^aktM) == 0) // (moj bit)^aktM jest ok
{
dp[v][j][0] = res;
}
else
{
res += maxdif;
dp[v][j][0] = res;
}
//dp[v][j][1] - uruchamiam u siebie
maxdif =maxn+13 , aktM = 0;
res = 1;
for(auto p : t[v])
{
if(p == o)continue;
res += min(dp[p][1][0],dp[p][1][1]);
if(dp[p][1][1] < dp[p][1][0])
{
aktM ^= 1;
}
maxdif= min(maxdif , max(dp[p][1][0],dp[p][1][1])-min(dp[p][1][0],dp[p][1][1]));
}
if(((j^stp[v]^1)^aktM) == 0) // (moj bit)^aktM jest ok
{
dp[v][j][1] = res;
}
else
{
res += maxdif;
dp[v][j][1] = res;
}
dp[v][j][0] = min(dp[v][j][0] , maxn+141);
dp[v][j][1] = min(dp[v][j][1] , maxn+141);
}
}
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n ,a , b;
cin >> n;
for(int i = 0 ; i < n-1 ; i++)
{
cin >> a >> b;
t[a].push_back(b);
t[b].push_back(a);
}
for(int i = 1; i <= n; i++)cin >> stp[i];
DFS(1,0);
int Ri = min(dp[1][0][0] , dp[1][0][1]);
if(Ri > maxn)cout << "impossible\n";
else cout << Ri << '\n';
return 0;
}
