package main

import (
	"fmt"
	"log"
)

func main() {
	log.SetFlags(log.Lshortfile)
	var N, K int
	fmt.Scan(&N, &K)
	var ans int
	if K == 0 || K > N {
		ans = 0
	} else if N%2 == 1 && N/2 == K-1 {
		ans = N - 1
	} else {
		ans = N - 2
	}
	fmt.Println(ans)
}