#include <bits/stdc++.h>

using namespace std;

using lint = long long;

using pint = pair<int, int>;

using vi = vector<int>;

#define rep(i, n) for(int i = 0; i < (int)(n); i++)

#define all(v) v.begin(), v.end()

int main() {
  int n, k;
  cin >> n >> k;
  rep(i, n-1) {
    int a, b;
    cin >> a >> b;
  }

  if (k > n) cout << "-1" << endl;
  else cout << k-1 << endl;

  return 0;
}