#include <bits/stdc++.h>
using namespace std;
const long long maxn = 1e7 + 10;
map<long long, long long> m;
map<long long, long long>::iterator it;
int main() {
  ios::sync_with_stdio(false);
  long long n, k;
  cin >> n >> k;
  for (long long i = 0; i < n; i++) {
    long long x;
    cin >> x;
    m[x]++;
  }
  long long vis = 0;
  long long flag = -1;
  it = m.begin();
  if (k == 0) {
    if (it->first - 1 >= 1 && it->first - 1 <= 1e9) {
      cout << it->first - 1 << endl;
      return 0;
    } else {
      cout << -1 << endl;
      return 0;
    }
  }
  for (; it != m.end(); it++) {
    vis += (it->second);
    if (vis == k && it->first >= 1 && it->first <= 1e9) {
      flag = it->first;
      break;
    }
    if (vis > k) break;
  }
  cout << flag << endl;
  return 0;
}
