#include<stdio.h>
int main()
{
	int n, m;
	scanf("%d %d", &n, &m);
	if (n % 2 > 0)
	{
		if (n <= m)
			printf("1\n");
		else
			printf("-1\n");
	}
	else
	{
		if (n <= m)
			printf("1\n");
		else if (n / 2 <= m)
			printf("2\n");
		else
			printf("-1\n");
	}
	return 0;
}