import java.util.*;

public class Main {
    private static Scanner sc = new Scanner(System.in);
    public static void main(String[] args) throws Exception {
        long n = sc.nextLong();
        long d = sc.nextLong();
        
        long ret;
        
        if (n==d) {
            ret = 0;
        } else {
            if (d>n/2) d = n-d;
            long m = n%d;
            if (m==0) {
                ret = n/d-1;
            } else {
                ret = n-1;
            }
        }
        
        System.out.println(ret);
    }
}