#include <bits/stdc++.h>
using namespace std;
#define int long long
#define all(x) (x).begin(), (x).end()
template<template<typename> class Container, typename G>
ostream& operator<<(ostream& os, Container<G> x) {
int f = 0; os << '{'; for (auto &i: x) os << (f++ ? ", " : ""), os << i; os << "}";
return os;
}
void _print() {cerr << "]\n";}
template<typename T, typename... V>
void _print(T t, V... v) {cerr << t; if (sizeof...(v)) cerr <<", "; _print(v...);}
#ifdef DEBUG
#define dbg(x...) cerr << "\e[91m"<<__func__<<":"<<__LINE__<<" [" << #x << "] = ["; _print(x); cerr << "\e[39m" << endl;
#else
#define dbg(x...)
#endif
#define pii pair<int, int>
#define f first
#define s second
vector<int> adj[100005];
int dp[100005][2][2];
int32_t main() {
int n;cin>>n;
for(int i=0;i<n-1;i++){
int u,v;cin>>u>>v;adj[u].emplace_back(v);adj[v].emplace_back(u);
}
vector<int>a(n+3);for(int i=1;i<=n;i++)cin>>a[i];
function<void(int,int)>dfs=[&](int u,int p){
for(int i=0;i<2;i++){
dp[u][a[u]^i][i]=i;
dp[u][a[u]^1ll^i][i]=1e9; // not right amount of stuff to have parent and this dude
}
int a=0,b=0; // a is pressing us, b is not pressing us
// dp[u][0][0] is pressing neither of us which means kid must have a parity of
for(int i:adj[u]){
if(i==p)continue;
dfs(i,u);
for(int j=0;j<2;j++){
// j is whether u is being pressed
a=dp[u][0][j],b=dp[u][1][j];
dp[u][0][j] = min(dp[i][j][0]+a, b+dp[i][j][1]); // j is whether or not we are pressing u, if we press a parent we gotta press the kid as well no?
dp[u][1][j] = min(dp[i][j][1]+a, b+dp[i][j][0]);
}
}
};
dfs(1,0);
int ans=min(dp[1][0][0],dp[1][0][1]);
if(ans>n){
cout<<"impossible\n";
}else{
cout<<ans<<"\n";
}
}
