#!ruby
n,k,*a=gets(p).split.map(&:to_i)
i=0
d=n/k
m=a[k-1]
2.upto(d) do
  u = (1..d).map{|i|a[i*k-1,k].min}.inject{|x,y|x+y}
  m = [m,u].min
end
p m