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

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