#include <bits/stdc++.h>
using namespace std;

int main( int argc, char *argv[] )
{
int n;
int m;

	ios::sync_with_stdio( false );
	cin.tie( 0 );

	cin >> n;
	cin >> m;
	if( n <= m )
		cout << 1 << endl;
	else if( n % 2 == 0 && n <= 2 * m )
		cout << 2 << endl;
	else
		cout << - 1 << endl;

	return 0;
}