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

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i=(a);i<(b);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
#define pb push_back
#define pf push_front
#define fi first
#define se second
#define BIT(x,n) bitset<n>(x)
#define PI 3.14159265358979323846

typedef long long ll;
typedef pair<int,ll> P;
typedef pair<ll,P> PP;

//-----------------------------------------------------------------------------

int n,m;
ll a[100000];

bool ok(int mid) {
	int l=1,r=n-1,cnt=0;
	while(l<r) {
		if(l==mid) {
			l++;
			continue;
		}
		if(r==mid) {
			r--;
			continue;
		}
		if(a[l]+a[r]>a[0]+a[mid]) l++,r--,cnt++;
		else l++;
	}
	//cout<<mid<<" "<<cnt<<endl;
	return cnt<m;
}

int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);

	cin>>n>>m;
	REP(i,n) cin>>a[i];
	sort(a+1,a+n);

	int ub=n,lb=0;
	while(ub-lb>1) {
		int mid=(ub+lb)/2;
		if(ok(mid)) ub=mid;
		else lb=mid;
	}
	if(ub==n) cout<<-1<<endl;
	else cout<<a[ub]<<endl;

	return 0;
}