package main

import "fmt"

func main() {
	var n, k int
	fmt.Scanf("%d %d", &n, &k)
	c := solve(n, k)
	fmt.Println(c)
}

func solve(n, k int) int {
	if n < 3 || k == 0 || k > n {
		return 0
	}
	if k*2-1 == n {
		return n - 1
	}
	return n - 2
}