#include <bits/stdc++.h>
const int Mod = 1e9 + 7, N = 1e5 + 1;
using namespace std;
using namespace std;
int cnt(int arr[], int targ, int n) {
  int cnt = 0;
  for (int i = 0; i < n; ++i) {
    if (arr[i] <= targ) {
      cnt++;
    }
  }
  return cnt;
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  int n, k;
  cin >> n >> k;
  int arr[n];
  for (int(i) = 0; (i) < (n); (i)++) {
    cin >> (arr)[i];
  };
  sort(arr, arr + n);
  if (k == 0) {
    if (arr[0] == 1) {
      cout << -1 << endl;
    } else {
      cout << arr[0] - 1 << endl;
    }
  } else {
    if (cnt(arr, arr[k - 1], n) > k || k == 0) {
      cout << -1 << endl;
    } else {
      if (arr[k - 1] == 1e9) {
        cout << 1000000000 << endl;
      } else {
        cout << arr[k - 1];
      }
    }
  }
}
