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 (n < k) {
		    System.out.println(-1);
		} else {
		    System.out.println(k - 1);
		}
	}
}