#include <bits/stdc++.h>
#define int long long 
using namespace std;
using Graph = vector<vector<int>>;

signed main(){
  int N, K;
  cin >> N >> K;
  Graph G(N);
  for( int i = 0; i < N-1; i++ ){
    int A, B;
    cin >> A >> B;
    A--;
    B--;
    G.at(A).push_back(B);
    G.at(B).push_back(A);
  }
  if( N < K ) cout << -1 << endl;
  else cout << K-1 << endl;
}