#include <bits/stdc++.h>

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define RFOR(i, a, b) for (int i = (b)-1; i >= (a); i--)
#define rep(i, n) for (int i = 0; i < (n); i++)
#define rep1(i, n) for (int i = 1; i <= (n); i++)
#define rrep(i, n) for (int i = (n)-1; i >= 0; i--)

#define pb push_back
#define mp make_pair
#define fst first
#define snd second
#define show(x) cout << #x << " = " << x << endl
#define chmin(x, y) x = min(x, y)
#define chmax(x, y) x = max(x, y)
#define pii pair<int, int>
#define vi vector<int>

using namespace std;
template <class S, class T>
ostream& operator<<(ostream& o, const pair<S, T>& p)
{
    return o << "(" << p.first << "," << p.second << ")";
}
template <class T>
ostream& operator<<(ostream& o, const vector<T>& vc)
{
    o << "sz = " << vc.size() << endl
      << "[";
    for (const T& v : vc)
        o << v << ",";
    o << "]";
    return o;
}
using ll = long long;
constexpr ll MOD = 1000000007;

int N, M;
constexpr int MAX = 100000;
ll a[MAX];

bool ok(int n)
{
    ll sum = a[0] + a[n];
    int cnt = 0;
    int head = 1;
    for (int i = N - 1; i >= 1; i--) {
        if (i == n) {
            continue;
        }
        while (head < i) {
            if (head == n) {
                head++;
                continue;
            }
            if (a[i] + a[head] > sum) {
                cnt++;
                head++;
                break;
            } else {
                head++;
            }
        }
    }
    if (cnt >= M) {
        return false;
    } else {
        return true;
    }
}

int main()
{
    cin >> N >> M;
    rep(i, N)
    {
        cin >> a[i];
    }
    sort(a + 1, a + N);
    int sup = N - 1;
    int inf = 1;
    if (not ok(sup)) {
        cout << -1 << endl;
        return 0;
    }
    if (ok(inf)) {
        cout << a[1] << endl;
        return 0;
    }
    while (inf < sup) {
        const int mid = (inf + sup) / 2;
        if (ok(mid)) {
            sup = mid;
        } else {
            if (inf == mid) {
                break;
            }
            inf = mid;
        }
    }
    cout << a[sup] << endl;

    return 0;
}