#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  int n, k;
  cin >> n >> k;
  int t[n];
  for (int i = 0; i < n; i++) cin >> t[i];
  sort(t, t + n);
  if (k == n)
    cout << t[n - 1] << endl;
  else if (k == 0) {
    if (t[0] - 1 == 0)
      cout << -1 << endl;
    else
      cout << t[0] - 1 << endl;
  } else if (k == 1) {
    if (t[0] == t[1])
      cout << -1 << endl;
    else
      cout << t[0] << endl;
  } else if (t[k - 1] == t[k])
    cout << -1 << endl;
  else
    cout << t[k - 1] << endl;
}
