#include "iostream"
using namespace std;

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