import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Scanner;
import java.util.TreeSet;

public class Main {
	
	public static int RANGE = 100;
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		final long N = sc.nextLong();
		final long K = sc.nextLong();
		
		if(K <= 0 || K > N){
			System.out.println(0);
		}else if(N % 2 == 1 && (K - 1) == (N / 2)){
			System.out.println(N - 1);
		}else{
			System.out.println(N - 2);
		}
	}
}