#include<iostream>
#include<string>
#include<algorithm>
#include<cmath>
#include<iomanip>
#include<cstring>
#include<map>
#include<vector>
#include<queue>
#include<climits>
using namespace std;
typedef long long int ll;

int main(){
	
	int n, m;
	cin >> n >> m;

	if(n<=m){
		cout << 1 << endl;
	} else if(n%2==0 && n/2<=m){
		cout << 2 << endl;
	} else {
		cout << -1 << endl;
	}

  return 0;
}