import java.util.*; public class Main { public static void main(String[] args) { Scanner sc = new Scanner(System.in); int N = sc.nextInt(); int K = sc.nextInt(); if(K<=0 || K>N){ System.out.println(0); } else if(N%2 == 0){ System.out.println(N-2); } else{ if( (K+1) == (N/2) ){ System.out.println(N-1); } else{ System.out.println(N-2); } } } }