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