#include<bits/stdc++.h>
//#define int long long
//#define int __int128
using namespace std;
inline int read()
{
   int x=0,f=1;char ch=getchar();
   while (ch<'0'||ch>'9'){if (ch=='-') f=-1;ch=getchar();}
   while (ch>='0'&&ch<='9'){x=(x<<3)+(x<<1)+ch-48;ch=getchar();}
   return x*f;
}
void write(int x)
{
   if(x<0)putchar('-'),x=-x;
   if(x<10)putchar(x+'0');
   else write(x/10),putchar(x%10+'0');
}
const int N=3e5;
const int mod=1e9+7;
int n,m;
int a[N]; 
int ans;
bool check(int x){
	int sum=a[1]+a[x];
	int r=n;
	int l=2;
	int sm=0;
	while(l<=r){
		if(l==x)l++;
		if(r==x)r--;
		if(a[r]+a[l]>sum)r--,sm++;
		l++;
	}
	return sm+1<=m;
}
signed main(){
//	freopen("prize.in","r",stdin);
//	freopen("prize.out","w",stdout);
	n=read(),m=read();
	for(int i=1;i<=n;i++)a[i]=read();
	sort(a+2,a+1+n);
	int l=2,r=n;
	while(l<=r){
		int mid=(l+r)>>1;
		if(check(mid))r=mid-1,ans=mid;
		else l=mid+1;
	}
	if(ans==0)cout<<-1<<"\n";
	else cout<<a[ans]<<"\n";
  return 0;
}