#include <bits/stdc++.h>
using namespace std;
#define int long long
#define vo vector
#define pb push_back
#define se second
#define fi first
#define all(v) v.begin(), v.end()
typedef vector<int> vi;
typedef pair<int, int> pii;
#define umap unordered_map
#define uset unordered_set
#define rep(i, a, b) for(int i=(a); i<b; i++)
#define pr1(x) cerr << #x << '=' << x << ' ';
//for google contests
#define repd(i, a, b) for(int i=(b-1); i >= a; i--)
void _pr(signed x) {cerr << x;}
void _pr(long long x) {cerr << x;}
void _pr(unsigned long long x) {cerr << x;}
void _pr(double x) {cerr << x;}
void _pr(char x) {cerr << '\'' << x << '\'';}
void _pr(const char* x) {cerr << x;}
void _pr(bool x) {cerr << (x ? "true" : "false");}
template<typename T, typename V> void _pr(const pair<T, V> &x);
template<typename T, typename V> void _pr(const pair<T, V> &x) {cerr << "\e[95m" << "[ "; _pr(x.first); cerr << ", "; _pr(x.second); cerr << "\e[95m" << ']';}
template<typename T> void _pr(const T &x) {int F=0; cerr << '{'; for(auto &i: x) cerr << (F++ ? ", " : ""), _pr(i); cerr << "\e[91m" << '}';}
template <typename T, typename... V> void _pr(T t, V... v) {_pr(t); if(sizeof...(v)) cerr << ", "; _pr(v...);}
#define pr(x...) cerr << "\e[91m" << __func__ << ':' << __LINE__ << " [" << #x << "] = ["; _pr(x); cerr << "\e[91m" << ']' << "\033[0m" << endl;
//go for outline with ;, then details
int const inf = 1e9, mxn = 1e5+4;
int n, ans=inf;
vo<vi> adj(mxn);
vi col, ini, off;
void press(int at){
col[at] = 1-col[at];
for(auto x: adj[at]){
col[x] = 1-col[x];
}
}
void sol1(){
rep(mask, 0, (1<<n)){
int ct = 0;
col = ini;
rep(i, 0, n){
if(mask & (1<<i)){
press(i);
ct++;
}
}
if(off == col) ans = min(ans, ct);
}
}
void sol3(){
int ct = 0;
rep(i, 1, n){
if(col[i-1]){
col[i-1] = 0; col[i] ^= 1;
if(i<n-1) col[i+1] ^= 1;
ct++;
}
if(i==n-1 && !col[i]) ans = min(ans, ct);
}
ct = 0;
col = ini; col[0] ^= 1; col[1] ^= 1;
rep(i, 1, n){
if(col[i-1]){
col[i-1] = 0; col[i] ^= 1;
if(i<n-1) col[i+1] ^= 1;
ct++;
}
if(i==n-1 && !col[i]) ans = min(ans, ct);
}
}
vi dfs(int at, int p){ //press and my color after press
vi ret = {inf, inf, inf, inf};
vo<vi> kids;
for(auto x: adj[at]){
if(x==p) continue;
vi tmp = dfs(x, at);
kids.pb(tmp);
}
if(kids.size() == 0){
if(ini[at]) ret = {1, inf, inf, 0};
else ret = {inf, 1, 0, inf};
}
else if(kids.size() == 1){
if(ini[at]){
ret = {min(inf, kids[0][3]+1), min(inf, kids[0][1]+1), kids[0][0], kids[0][2]};
}
else{
ret = {min(inf, kids[0][1]+1), min(inf, kids[0][3]+1), kids[0][2], kids[0][0]};
}
}
else{
vi a = kids[0], b = kids[1];
//press
for(int i: {1, 3}){
for(int u: {1, 3}){
if(a[i]==inf || b[u] == inf) continue;
int ind = ((ini[at]^1) + (i==1) + (u==1))%2;
ret[ind] = min(ret[ind], a[i] + b[u] + 1);
}
}
//dont press
for(int i: {0, 2}){
for(int u: {0, 2}){
if(a[i]==inf && b[u] == inf) continue;
int ind = (ini[at] + (i==0) + (u==0))%2 + 2;
ret[ind] = min(ret[ind], a[i] + b[u]);
}
}
}
return ret;
}
vi full(int at, int p){
vi ret = {inf, inf, inf, inf};
vo<vi> kids;
for(auto x: adj[at]){
if(x==p) continue;
vi tmp = full(x, at);
kids.pb(tmp);
}
if(kids.size() == 0){
if(ini[at]) ret = {1, inf, inf, 0};
else ret = {inf, 1, 0, inf};
}
else{
//find cheapest for all children and swap one to get reverse color
//press so all kids white
int sum = 1, posi = 1, ind = 0, cheap = inf;
for(auto v: kids){
if(v[3] > v[1]) {
sum += v[1];
ind++;
if(v[3]!=inf) cheap = min(cheap, v[3]-v[1]);
}
else{
if(v[3] == v[1] && v[1] == inf) {posi = 0; break;}
sum += v[3];
if(v[1] != inf) cheap = min(cheap, v[1]-v[3]);
}
}
if(posi){
ind = ((ini[at]^1) + ind)%2;
ret[ind] = sum;
if(cheap != inf) ret[ind^1] = sum + cheap;
}
//dont press
sum = 0, posi = 1, ind = 0, cheap = inf;
for(auto v: kids){
if(v[2] > v[0]) {
sum += v[0];
ind++;
if(v[2] != inf) cheap = min(cheap, v[2]-v[0]);
}
else{
if(v[2] == v[0] && v[0] == inf) {posi = 0; break;}
sum += v[2];
if(v[0] != inf) cheap = min(cheap, v[0]-v[2]);
}
}
if(posi){
ind = (ini[at] + ind)%2;
ret[ind+2] = sum;
if(cheap != inf) ret[(ind^1) + 2] = sum + cheap;
}
}
return ret;
}
signed main(){
cin.tie(0)->sync_with_stdio(0);
cin>>n; col.resize(n), ini.resize(n), off.assign(n, 0);
vi indeg(n, 0);
int linegraph = 1;
rep(i, 0, n-1){
int a, b; cin>>a>>b; a--, --b;
adj[a].pb(b), adj[b].pb(a); indeg[a]++, indeg[b]++;
if(abs(a-b) > 1) linegraph = 0;
}
rep(i, 0, n) {
cin>>col[i]; ini[i] = col[i];
}
if(n<=20){
sol1();
}
// else if(n<=40) ;//press centroid and then do bitmask on each subtree
else if(linegraph){
sol3();
}
else if(0){
//binary tree
rep(i, 0, n){
if(indeg[i] == 1){
vi tmp = dfs(i, i);
ans = min(tmp[0], tmp[2]);
break;
}
}
}
else{
vi tmp = full(0, 0);
ans = min(tmp[0], tmp[2]);
}
(ans==inf) ? cout << "impossible" : cout << ans;
}
