#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<ll, ll> P;
#define DUMP(x) cout << #x << " = " << (x) << endl;
#define FOR(i, m, n) for (ll i = m; i < n; i++)
#define IFOR(i, m, n) for (ll i = n - 1; i >= m; i--)
#define REP(i, n) FOR(i, 0, n)
#define IREP(i, n) IFOR(i, 0, n)
#define FOREACH(x, a) for (auto&(x) : (a))
#define ALL(v) (v).begin(), (v).end()
#define SZ(x) ll(x.size())

ll n, k;
vector<vector<ll>> to;
vector<ll> dist;

void dfs(ll cv, ll par, ll d) {
  dist[cv] = d;
  FOREACH(nv, to[cv]) {
    if (nv == par) {
      continue;
    }
    dfs(nv, cv, d + 1);
  }
}

int main() {
  cin >> n >> k;
  to = vector<vector<ll>>(n);
  REP(i, n - 1) {
    ll a, b;
    cin >> a >> b;
    a--, b--;
    to[a].push_back(b);
    to[b].push_back(a);
  }

  if (n < k) {
    cout << -1 << endl;
    exit(0);
  }

  // dist = vector<ll>(n);
  // dfs(0, -1, 0);
  // sort(ALL(dist));

  cout << k - 1 << endl;
}