def fac(n)
  return (1..n).inject(:*) || 1
end

n, k = gets.split.map(&:to_i)

gt = n - k

if k == 1
  puts gt * (gt - 1) - (gt - 1)
else
  ans = 1
  1.upto(gt) do |i|
    ans += fac(gt) / fac(i) / fac(gt - i)
  end
  puts ans
end