#include<cstdio>


int main(void)
{
	int n, m;

	while(scanf("%d%d", &n, &m)==2)
	{
		if(n<=m) printf("1\n");
		else if(n%2==0 && n<=m*2) printf("2\n");
		else printf("-1\n");
	}
	return 0;
}