#include <bits/stdc++.h>
#define REP(i,n,N) for(int i=(n);i<(int)N;i++)
#define p(s) cout<<(s)<<endl
#define ck(n,a,b) ((a)<=(n)&&(n)<=(b))
#define F first
#define S second
typedef long long ll;
using namespace std;
const int inf=1e9;

int main() {
	int n,m;
	cin>>n>>m;
	if(n<=m) p(1);
	else if(n%2==0&&m*2>=n) p(2);
	else p(-1);
	return 0;
}