#include <bits/stdc++.h>
using namespace std;

#define rep(i, n) for(int(i)=0;(i)<(n);(i)++)

int main() {
  int n, k;
  cin >> n >> k;
  if(n < k) cout << -1 << endl;
  else {
    vector<vector<int>> G(n);
    rep(i, n-1) {
      int a, b;
      cin >> a >> b;
      a--;b--;
      G[a].emplace_back(b);
      G[b].emplace_back(a);
    }

    queue<int> que;
    vector<int> used(n);
    vector<int> depth(n);
    que.emplace(0);
    int num = 0;
    while(!que.empty()) {
      int v = que.front();que.pop();
      used[v] = 1;
      num++;
      if(num == k) break;
      for(auto next_v : G[v]) {
        if(used[next_v]) continue;
        depth[next_v] = depth[v] + 1;
        que.emplace(next_v);
      }
    }

    cout << *max_element(depth.begin(), depth.end()) << endl;
  }
}