eval"N,M="+gets.split*?,
x,*a=(1..N).map{gets.to_i};a.sort!
p a.bsearch{|y|b=a[...q=a.index(y)]+a[q+1..];(N-3).downto(i=0).count{|j|i+=1while i<j&&x+y>=b[i]+b[j];i<j&&i+=1}<M}||-1