479_C

#include <iostream>
#include <algorithm>
using namespace std;
int main() {
    int n, k;
    cin >> n >> k;
    long long a[n];
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }
    sort(a, a + n);
    if (k == 0) {
        cout << (a[0] == 1 ? -1 : 1);
        return 0;
    }
    if ((k < n && a[k] > a[k - 1]) || k == n) {
        cout << a[k - 1] << endl;
    } else {
        cout << -1 << endl;
    }
}