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

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

ans = (k == 1) ? 0 : 1

gt = n - k
1.upto(gt) do |i|
  ans += fac(gt) / fac(i) / fac(gt - i)
end

if n == 2 and k == 1
  puts 0
else
  puts ans
end