package yukicoder;

import java.util.Arrays;
import java.util.Scanner;

public class N507
{
	static int[] a,buf;
	static int k,n,m,ans;
	public static void main(String[] args)
	{
		Scanner sc = new Scanner(System.in);
		n=sc.nextInt();
		m=sc.nextInt();
		int min=0,max=n-2;
		a=new int[n-1];
		k=sc.nextInt();//k a[0]-a[n-2]
		for(int i=0;i<n-1;i++)
		{
			a[i]=sc.nextInt();
		}
		Arrays.sort(a);

		if(isok(min)){ans=min;}
		else if(!isok(max)){ans=-1;}
		else
		{
			ans=(int)Math.ceil((min+max)/2);
			while(max-min>1)
			{
				//System.out.println("min max"+min+" "+max);
				ans=(int)Math.ceil((min+max)/2);
				if(isok(ans)){max=ans;}
				else{min=ans;}
			}
		}
		System.out.println(ans);



	}
	static boolean isok(int ind)//
	{
		int me=k+a[ind];
		int rank=1,buf=0;
		int end=0;

		for(int i=n-2;i>=0;i--)
		{
			if(ind==i){continue;}
			for(int j=buf;j<i;j++)
			{
				if(ind==j){continue;}
				buf=j;
				if(me<a[i]+a[j]){rank++;break;}
			}
		}
		return rank<=m;
	}
}