#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define ull unsigned long long
#define pb(e) push_back(e)
#define sv(a) sort(a.begin(),a.end())
#define sa(a,n) sort(a,a+n)
#define mp(a,b) make_pair(a,b)
#define vf first
#define vs second
#define ar array
#define all(x) x.begin(),x.end()
const int inf = 0x3f3f3f3f;
const int mod = 1000000007;
const double PI=3.14159265358979323846264338327950288419716939937510582097494459230;
mt19937_64 RNG(chrono::steady_clock::now().time_since_epoch().count());
bool remender(ll a , ll b){return a%b;}
//freopen("problemname.in", "r", stdin);
//freopen("problemname.out", "w", stdout);
const int N = 200002;
int n , k;
vector<int> adj[N];
int col[N];
bool vis[N],vis_col[N];
vector<int> v[N];
int sub[N];
int ans;
int p[N];
int done[N];
void pre(int node , int par){
sub[node] = 1;
for(int i : adj[node]){
if(!vis[i] && i != par){
pre(i , node);
sub[node] += sub[i];
}
}
}
int find(int node , int par , int half){
for(int i : adj[node]){
if(!vis[i] && i != par && sub[i] > half){
return find(i , node , half);
}
}
return node;
}
int counting(int node , int par , int c , int centroid){
p[node] = par;
done[node] = centroid;
int cnt = 0;
for(int i : adj[node]){
if(!vis[i] && i != par){
cnt += counting(i , node , c , centroid);
}
}
if(col[node] == c)cnt++;
return cnt;
}
void dfs(int node , int par){
done[node] = 0;
for(int i : adj[node]){
if(!vis[i] && i != par){
dfs(i , node);
}
}
}
void centroid_decomposition(int node){
pre(node , node);
int centroid = find(node , node , sub[node]/2);
int c = col[centroid];
int cnt = counting(centroid,centroid,c ,-centroid);
if(cnt == v[c].size()){
int res = 0;
queue<int> q;
for(int i : v[c]){
done[i] = 1;
q.push(i);
}
while(q.size()){
node = q.front();
q.pop();
if(node == centroid)continue;
int par = p[node];
if(done[v[col[par]][0]] == -centroid){
if(vis_col[col[par]]){
res = -1;
break;
}
res++;
for(int i : v[col[par]]){
if(done[i] != -centroid){
res = -1;
break;
}
done[i] = 1;
q.push(i);
}
}
else if(done[v[col[par]][0]] == 0){
res = -1;
break;
}
}
if(res != -1){
ans = min(ans , res);
}
}
dfs(centroid , centroid);
vis_col[c] = 1;
vis[centroid] = 1;
for(int i : adj[centroid]){
if(!vis[i]){
centroid_decomposition(i);
}
}
}
void solve(){
cin >> n;
cin >> k;
for(int i = 0; i < n-1; i++){
int a , b;
cin >> a >> b;
adj[a].pb(b);
adj[b].pb(a);
}
for(int i = 1; i <= n; i++){
cin >> col[i];
v[col[i]].pb(i);
}
memset(vis,0,sizeof vis);
memset(vis_col,0,sizeof vis_col);
memset(done,0,sizeof done);
ans = k-1;
centroid_decomposition(1);
cout << ans << '\n';
}
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
//int t;cin >> t;while(t--)
solve();
return 0;
}
