#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define ii pair<int, int>
#define f first
#define s second
#define pb push_back
#define mp make_pair
#define all(x) x.begin(), x.end()
#define F0R(i, n) for (int i = 0; i < n; i++)
#define FOR(i, a, b) for (int i = a; i < b; i++)
#define inf 1000000010
#define maxn 300000
int n, m;
vector<int> adj[maxn];
bool lit[maxn];
int need[maxn]; // # of dynamites needed
int farthestDyn[maxn]; // farthest dynamite in subtree of i
int extraExplosion[maxn]; // extra explosion coming out of subtree of i
void dfs(int u, int p, int t) {
for (int v : adj[u]) {
if (v != p) {
dfs(v, u, t);
}
}
int extra = 0;
for (int v : adj[u]) {
if (v == p) continue;
extra = max(extra, extraExplosion[v]);
}
--extra;
int farDyn = lit[u] ? 0 : -inf;
for (int v : adj[u]) {
if (v == p) continue;
farDyn = max(farDyn, farthestDyn[v] + 1);
}
if (farDyn <= extra) {
farDyn = -inf;
}
assert(farDyn <= t);
int totNeed = 0;
for (int v : adj[u]) {
if (v == p) continue;
totNeed += need[v];
}
if (farDyn == t) {
farDyn = -inf;
extra = t;
totNeed++;
}
need[u] = totNeed;
farthestDyn[u] = farDyn;
extraExplosion[u] = extra;
}
int main() {
cin.tie(0)->sync_with_stdio(0);
cin >> n >> m;
F0R(i, n) cin >> lit[i];
F0R(i, n-1) {
int a, b; cin >> a >> b; --a; --b;
adj[a].pb(b);
adj[b].pb(a);
}
int lo = 0, hi = n, mid, ans = -1;
while (lo < hi) {
mid = (lo+hi)/2;
dfs(0, 0, mid);
if (farthestDyn[0] >= 0) need[0]++;
if (need[0] <= m) {
hi = mid;
ans = mid;
} else {
lo = mid+1;
}
}
cout << ans << endl;
return 0;
}
