// EXPLOSION!
#define _CRT_SECURE_NO_WARNINGS
#include<bits/stdc++.h>
#include<unordered_set>
#include<unordered_map>
#include<chrono>
using namespace std;
typedef pair<int, int> pii;
typedef long long ll;
typedef pair<ll, ll> pll;
typedef long double ld;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<pair<int, int>> vpi;
typedef vector<pair<ll, ll>> vpll;
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define F0R(i,a) FOR(i,0,a)
#define ROF(i,a,b) for (int i = (b)-1; i >= (a); --i)
#define R0F(i,a) ROF(i,0,a)
#define trav(a,x) for (auto& a: x)
#define pb push_back
#define mp make_pair
#define rsz resize
#define sz(x) int(x.size())
#define all(x) x.begin(),x.end()
#define f first
#define s second
#define cont continue
#define endl '\n'
//#define ednl '\n'
#define test int testc;cin>>testc;while(testc--)
#define pr(a, b) trav(x,a)cerr << x << b; cerr << endl;
#define message cout << "Hello World" << endl;
const int dx[4] = { 1,0,-1,0 }, dy[4] = { 0,1,0,-1 }; // for every grid problem!!
const ll linf = 4000000000000000000LL;
const ll inf = 1000000007;//998244353
void pv(vi a) { trav(x, a)cout << x << " "; cout << endl; }void pv(vll a) { trav(x, a)cout << x << " "; cout << endl; }void pv(vector<vi>a) {
F0R(i, sz(a)) { cout << i << endl; pv(a[i]); cout << endl; }
}void pv(vector<vll>a) { F0R(i, sz(a)) { cout << i << endl; pv(a[i]); }cout << endl; }void pv(vector<string>a) { trav(x, a)cout << x << endl; cout << endl; }
void setIO(string s) {
ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef arwaeystoamneg
if (sz(s))
{
freopen((s + ".in").c_str(), "r", stdin);
if (s != "test1")
freopen((s + ".out").c_str(), "w", stdout);
}
#endif
}
const int MAX = 111111;
int n;
vector<vi>adj;
vi a;
int dp[MAX][2][2];
void dfs(int i, int p = -1)
{
if (i && sz(adj[i]) == 1)
{
dp[i][a[i]][0] = 0;
dp[i][1 ^ a[i]][1] = 1;
return;
}
trav(x, adj[i])
{
if (x == p)continue;
dfs(x, i);
}
// if do not toggle point
int cost[2] = { inf,inf }, ne[2] = { 0,0 };
cost[a[i]] = 0;
trav(x, adj[i])
{
if (x == p)continue;
ne[0] = ne[1] = inf;
F0R(j, 2)
{
ne[j] = min(ne[j], cost[j] + dp[x][0][0]);
ne[j ^ 1] = min(ne[j ^ 1], cost[j] + dp[x][0][1]);
}
cost[0] = ne[0], cost[1] = ne[1];
}
dp[i][0][0] = cost[0];
dp[i][1][0] = cost[1];
// toggle point
cost[0] = cost[1] = inf;
cost[a[i]] = 1;
trav(x, adj[i])
{
if (x == p)continue;
ne[0] = ne[1] = inf;
F0R(j, 2)
{
ne[j] = min(ne[j], cost[j] + dp[x][1][0]);
ne[j ^ 1] = min(ne[j ^ 1], cost[j] + dp[x][1][1]);
}
cost[0] = ne[0], cost[1] = ne[1];
}
dp[i][1][1] = cost[0];
dp[i][0][1] = cost[1];
}
int main()
{
setIO("test1");
cin >> n;
a.rsz(n);
adj.rsz(n);
F0R(i, n - 1)
{
int x, y;
cin >> x >> y;
x--, y--;
adj[x].pb(y);
adj[y].pb(x);
}
trav(x, a)cin >> x;
F0R(i, n)dp[i][0][0] = dp[i][0][1] = dp[i][1][0] = dp[i][1][1] = inf;
dfs(0);
int ans = min(dp[0][0][0], dp[0][0][1]);
if (ans > n)cout << "impossible" << endl;
else cout << ans << endl;
}
