#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<string>
#include<iostream>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include <sstream>
#include<iomanip>
#include <ctype.h>
using namespace std;

//#include<bits/stdc++.h>


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

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