#include <bits/stdc++.h>
#define pb push_back
#define f first
#define sc second
using namespace std;
typedef long long int ll;
typedef string str;
const ll inf = 1e9;
int n;
vector<vector<int>> v;
vector<vector<vector<ll>>> dp;
vector<int> state;
void dfs(int nd, int ss){
if(v[nd].size() == 1 && ss != -1){
dp[nd][0][0] = (state[nd]?inf:0);
dp[nd][0][1] = (state[nd]?1:inf);
dp[nd][1][0] = (state[nd]?0:inf);
dp[nd][1][1] = (state[nd]?inf:1);
return;
}
for(int x: v[nd]) if(x != ss) dfs(x, nd);
dp[nd][0][1] = 1;
dp[nd][1][1] = 1;
int cnt = 0;
ll mn = inf;
for(int x: v[nd]) if(x != ss){
dp[nd][0][0] += min(dp[x][0][0], dp[x][0][1]);
dp[nd][1][0] += min(dp[x][0][0], dp[x][0][1]);
if(dp[x][0][0] > dp[x][0][1]) cnt++;
mn = min(mn, abs(dp[x][0][0]-dp[x][0][1]));
}
if(state[nd]){
if(cnt%2 == 0) dp[nd][0][0]+=mn;
else dp[nd][1][0]+=mn;
}
else{
if(cnt%2 == 0) dp[nd][1][0]+=mn;
else dp[nd][0][0]+=mn;
}
cnt = 0;
mn = inf;
for(int x: v[nd]) if(x != ss){
dp[nd][0][1] += min(dp[x][1][0], dp[x][1][1]);
dp[nd][1][1] += min(dp[x][1][0], dp[x][1][1]);
if(dp[x][1][0] > dp[x][1][1]) cnt++;
mn = min(mn, abs(dp[x][1][0]-dp[x][1][1]));
}
if(!state[nd]){
if(cnt%2 == 0) dp[nd][0][1]+=mn;
else dp[nd][1][1]+=mn;
}
else{
if(cnt%2 == 0) dp[nd][1][1]+=mn;
else dp[nd][0][1]+=mn;
}
dp[nd][0][0] = min(dp[nd][0][0], inf);
dp[nd][1][0] = min(dp[nd][1][0], inf);
dp[nd][0][1] = min(dp[nd][0][1], inf);
dp[nd][1][1] = min(dp[nd][1][1], inf);
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
cin >> n;
v.resize(n);
state.resize(n);
dp.assign(n, vector<vector<ll>>(2, vector<ll>(2, 0)));
for(int i = 0; i < n-1; i++){
int a, b; cin >> a >> b; a--, b--;
v[a].pb(b);
v[b].pb(a);
}
for(int &x: state) cin >> x;
dfs(0, -1);
ll ans = min(dp[0][0][0], dp[0][0][1]);
if(ans >= inf) cout << "impossible\n";
else cout << ans << "\n";
}
