func main()
	var n: int :: cui@inputInt()
	var k: int :: cui@inputInt()
	var ans: int :: 0
	if(k = 1)
		do ans :: (n - 2) * (n - 1) / 2
	else
		do ans :: 2 ^ (n - k)
	end if
	do cui@print("\{ans}\n")
end func