#include<bits/stdc++.h>
using namespace std;
#define rep(i,n) for(ll i=0;i<n;i++)
#define repl(i,l,r) for(ll i=(l);i<(r);i++)
#define per(i,n) for(ll i=(n)-1;i>=0;i--)
#define perl(i,r,l) for(ll i=r-1;i>=l;i--)
#define fi first
#define se second
#define pb push_back
#define ins insert
#define pqueue(x) priority_queue<x,vector<x>,greater<x>>
#define all(x) (x).begin(),(x).end()
#define CST(x) cout<<fixed<<setprecision(x)
#define rev(x) reverse(x);
using ll=long long;
using vl=vector<ll>;
using vvl=vector<vector<ll>>;
using pl=pair<ll,ll>;
using vpl=vector<pl>;
using vvpl=vector<vpl>;
const ll MOD=1000000007;
const ll MOD9=998244353;
const int inf=1e9+10;
const ll INF=4e18;
const ll dy[8]={1,0,-1,0,1,1,-1,-1};
const ll dx[8]={0,-1,0,1,1,-1,1,-1};
template<class T> inline bool chmin(T& a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template<class T> inline bool chmax(T& a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}
int main(){
    ll n,m;cin >> n >> m;
    ll f;cin >> f;
    vl v(n-1);rep(i,n-1)cin >> v[i];
    sort(all(v));
    ll ok=n-1,ng=-1;
    while(ok-ng>1){
        ll mid=(ok+ng)>>1;
        multiset<ll> st;
        ll score=f+v[mid];
        rep(i,n-1){
            if(i==mid)continue;
            st.insert(v[i]);
        }
        ll res=0;
        while(st.size()){
            ll r=*st.rbegin();st.erase(st.find(r));
            auto itr=st.upper_bound(score-r);
            if(itr==st.end())break;
            //cout << r <<" " << *itr << endl;
            res++;
            st.erase(itr);
        }
        if(res>=m)ng=mid;
        else ok=mid;
    }
    //cout << ok << endl;
    if(ok==n-1)cout << -1 << endl;
    else cout << v[ok] << endl;
}