#include<cstdio>
#include <iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>

using namespace std;
typedef long long ll;
typedef pair<int, int> Pr;

int N;
int a[17];
queue<int> q;

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

	int a = n / m;
	if (n % m != 0)a++;
	if (n == m || n%2 == 0){
		if (n <= m)
			cout << 1 << endl;
		else if (n / m == 2)
			cout << 2 << endl;
		else
			cout << -1 << endl;
	}
	else {
		cout << -1 << endl;
	}
	
	return	0;
}