import java.util.*;

public class T02 {
    /*
    int gcd(int a, int b) {
        if ( b > a) return gcd(b, a);
        else if (b == 0) return a;
        eklse
        */
    
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int D = sc.nextInt();
        if ( N / 2 < D) {
            D = N - D;
        }
        if (D == 0) {
            System.out.println(0);
        }
        else if (N % D == 0) {
            System.out.println(N / D - 1);
        } else {
            System.out.println(N - 1);
        }

    }

}