#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 6;
const int MAX = 1e7 + 7;
int n, k;
int a[N];
int dp[MAX];
bool can(int x) {
  memset(dp, 0, sizeof(dp));
  for (int i = x; i <= MAX; i++) {
    dp[i] = max(1, dp[i / 2] + dp[(i + 1) / 2]);
  }
  int kk = k;
  for (int i = 0; i < n; i++) {
    kk -= a[i] < x ? 0 : dp[a[i]];
    if (kk <= 0) {
      return true;
    }
  }
  return false;
}
int main() {
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  cin >> n >> k;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
  }
  int lo = 0, hi = 1e7 + 100;
  while (lo + 1 < hi) {
    int mid = (lo + hi) / 2;
    if (!can(mid)) {
      hi = mid;
    } else {
      lo = mid;
    }
  }
  cout << (lo == 0 ? -1 : lo) << endl;
  return 0;
}
