#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <cmath>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <map>
#include <functional>
#include <set>
#include <numeric>
#include <stack>
#include <utility>
#include <time.h>
//#include "util.h"

using namespace std;
typedef long long lint;
typedef unsigned long long ull;
typedef pair<lint, lint> Pii;

#define PI 3.14159265358979323846
#define EPS 1e-6
#define MOD ((lint)1000000007)
#define MIN(a,b) ((a)<(b)?(a):(b))
#define MAX(a,b) ((a)>(b)?(a):(b))
#define FOR(i,a,b) for (int i=(a);i<(b);i++)
#define RFOR(i,a,b) for (int i=(b)-1;i>=(a);i--)
#define REP(i,n) for (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)

template <typename _Ty>
ostream& operator << (ostream& ostr, const vector<_Ty>& v) {
	if (v.empty()) {
		cout << "{ }";
		return ostr;
	}
	cout << "{" << v.front();
	for (auto itr = ++v.begin(); itr != v.end(); itr++) {
		cout << ", " << *itr;
	}
	cout << "}";
	return ostr;
}

int countSetBits(int n)
{
	int cnt = 0;
	while (n) {
		cnt++;
		n &= n - 1;
	}
	return cnt;
}

bool yuki0531()
{
	int n, m;
	cin >> n >> m;

	if (n % 2) {
		if (n > m)
			cout << -1 << endl;
		else
			cout << 1 << endl;
	}
	else {
		if (2 * m < n)
			cout << -1 << endl;
		else if (m < n)
			cout << 2 << endl;
		else
			cout << 1 << endl;
	}

	return 0;
}

int main()
{
	//clock_t start, end;
	//start = clock();

	yuki0531();

	//end = clock();
	//printf("%d msec.\n", end - start);

	return 0;
}