#include <bits/stdc++.h>
  
using namespace std;
  
#define rep(i,n) REP(i,0,n)
#define REP(i,s,e) for(int i=(s); i<(int)(e); i++)
#define repr(i, n) REPR(i, n, 0)
#define REPR(i, s, e) for(int i=(int)(s-1); i>=(int)(e); i--)
#define pb push_back
#define all(r) r.begin(),r.end()
#define rall(r) r.rbegin(),r.rend()
#define fi first
#define se second
  
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
 
const int INF = 1e9;
const ll MOD = 1e9 + 7;
double EPS = 1e-8;

int main(){
    int N, M;
    cin >> N >> M;
    vl a(N);
    rep(i, N) cin >> a[i];
    ll x = a[0];
    vl y(N-1);
    rep(i, N-1) y[i] = a[i+1];
    sort(all(y));
    int l = -1, r = y.size()-1;
    auto check = [&](int m) {
        multiset<ll> st;
        rep(i, N-1) if(i != m) st.insert(y[i]);
        ll r = x + y[m];
        int cnt = 0;
        while(st.size() > 1) {
            auto a = *st.begin();
            st.erase(st.begin());
            auto it = st.upper_bound(r-a);
            if(it == st.end()) continue;
            ++cnt;
            st.erase(it);
        }
        // cout << m << " " << y[m] << " " << r << " " << cnt << endl;
        return cnt < M;
    };  
    while(r - l > 1) {
        int m = (r + l) / 2;
        if(check(m)) r = m;
        else l = m;
    }
    if(!check(r)) cout << -1 << '\n';
    else cout << y[r] << '\n';
    return 0;
}