#include "iostream"
#include "climits"
#include "list"
#include "queue"
#include "stack"
#include "set"
#include "functional"
#include "algorithm"
#include "math.h"
#include "utility"
#include "string"
#include "map"
#include "unordered_map"
#include "iomanip"
#include "random"

using namespace std;
const long long int MOD = 1000000007;
list<long long int> Prime(int M) {
	list<long long int>P;
	P.push_back(2);
	P.push_back(3);
	for (int i = 5; i <= M; i += 6) {
		bool flag = true;
		for (auto j : P) {
			if (i%j == 0) {
				flag = false;
				break;
			}
		}
		if (flag)P.push_back(i);
		flag = true;
		for (auto j : P) {
			if ((i + 2) % j == 0) {
				flag = false;
				break;
			}
		}
		if (flag)P.push_back(i + 2);
	}
	return P;
}
long long int power(long long int x, long long int n, long long int M) {
	long long int tmp = 1;

	if (n > 0) {
		tmp = power(x, n / 2, M);
		if (n % 2 == 0) tmp = (tmp*tmp) % M;
		else tmp = (((tmp*tmp) % M)*x) % M;
	}
	return tmp;
}

long long int N, M, K, Q, W, H;
long long int ans;

int main() {
	ios::sync_with_stdio(false);
	cin >> N >> M;
	if (N <= M) {
		cout << "1\n";
		return 0;
	}
	if (N % 2) {
		cout << "-1\n";
		return 0;
	}
	if (N > M * 2) {
		cout << "-1\n";
		return 0;
	}
	cout << "5\n";
	return 0;
}