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

#define int long long
#define rep(i, l, r) for (int i = (int)(l); i < (int)(r); i++)
#define all(x) (x).begin(), (x).end()
#define sz(x) ((int)x.size())
template <class T> bool chmax(T &a, T b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T> bool chmin(T &a, T b) {
  if (a > b) {
    a = b;
    return 1;
  }
  return 0;
}

template <class T> using V = vector<T>;
using P = pair<int, int>;

/*
 *  */

int n, k;

signed main() {
  cin >> n >> k;
  rep(i, 0, n-1){
    int a, b;
    cin >> a >> b;
    a--; b--;
  }

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

  return 0;
}