#include <iostream>
#include <vector>
using namespace std;

int main(){
  int N, K;
  cin >> N >> K;
  vector<vector<int>> G(N);
  for(int i = 1; i < N; ++i){
    int a, b;
    cin >> a >> b;
    --a,--b;
    G[a].push_back(b);
    G[b].push_back(a);
  }
  if(N < K){
    cout << -1 << endl;
    return 0;
  }
  cout << K-1 << endl;
}