#include <iostream>
#include <fstream>
#include <cstdio>
#include <cmath>
#include <vector>
#include <cstring>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <algorithm>
using namespace std;

#define REP(i,n) for(int i=0; i<n; ++i)
#define FOR(i,a,b) for(int i=a; i<=b; ++i)
#define FORR(i,a,b) for (int i=a; i>=b; --i)
#define ALL(c) (c).begin(), (c).end()

typedef long long ll;
typedef vector<int> VI;
typedef vector<ll> VL;
typedef vector<VI> VVI;
typedef pair<int,int> P;
typedef pair<ll,ll> PL;

int main() {
	int n, m;
	cin >> n >> m;
	if (n <= 3 && m <= n-1) {
		cout << -1 << endl;
		return 0;
	}
	int ans;
	if (m >= n) ans = 1;
	else{
		if (n % 2 == 0) ans = (n - m + 1) / m;
		else ans = max(3, (n-m+1) / m);
	}
	cout << ans << endl;

    return 0;
}