#include<stdio.h>

int main(){
	int n,m;
	scanf("%d %d",&n,&m);
	int n2=n/2+n%2;
	if(n2>m){
		printf("-1\n");
	}else if(n%2==1&&n>m){
		printf("-1\n");
	}else{
		if(n<=m){
			printf("1\n");
		}else{
			printf("2\n");
		}
	}
}