#include<iostream>
#include<algorithm>
#include<vector>
#include<functional>
#include<string>
#include<cmath>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<cstdio>
using namespace std;
using ll = long long;

using Graph = vector<vector<int>>;
int main(){
  ll n, k;
  cin >> n >> k;
  vector<ll> a(n), b(n);
  //Graph g(n * n);

  for (int i = 0; i < n - 1; i++){
    cin >> a[i] >> b[i];
    //g[a[i]].push_back(b[i]);
    //g[b[i]].push_back(a[i]);
  }
  if(n < k){
    cout << -1 << "\n";
    return 0;
  }
  cout << k - 1 << "\n";

  return 0;
}