func main()
	var n: int :: cui@inputInt()
	var k: int :: cui@inputInt()
	if(k = 0 | k > n)
		do cui@print("0\n")
		ret
	end if
	
	var ans: int :: n - ((n + 1 - k = k) ?(1, 2))
	do cui@print("\{ans}\n")
end func