#include<bits/stdc++.h>
using namespace std;

#define int long long

#define rep(i,n) for(int i=0;i<(n);i++)
#define pb push_back
#define all(v) (v).begin(),(v).end()
#define fi first
#define se second
typedef vector<int>vint;
typedef pair<int,int>pint;
typedef vector<pint>vpint;

template<typename A,typename B>inline void chmin(A &a,B b){if(a>b)a=b;}
template<typename A,typename B>inline void chmax(A &a,B b){if(a<b)a=b;}

int N,M;
int A[111111];

bool C(int x){
    multiset<int>s;
    rep(i,N){
        if(i==0||i==x)continue;
        s.insert(A[i]);
    }

    int X=A[0]+A[x];

    int cnt=0;
    while(s.size()){
        int t=*s.rbegin();
        auto it=s.end();
        it--;
        s.erase(it);

        it=s.upper_bound(X-t);
        if(it==s.end())break;
        cnt++;
        s.erase(it);
    }

    return cnt<M;
}

signed main(){
    cin>>N>>M;
    rep(i,N)cin>>A[i];

    sort(A+1,A+N);
    int lb=0,ub=N;
    while(ub-lb>1){
        int mid=(ub+lb)/2;
        if(C(mid))ub=mid;
        else lb=mid;
    }

    if(ub==N)cout<<-1<<endl;
    else cout<<A[ub]<<endl;
    return 0;
}