#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

int N, K;
vector<int> G[101010];

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

  return 0;
}