#include <iostream>
#include <iomanip>
#include <stack>
#include <queue>
#include <vector>
#include <algorithm>
#include <map>
#include <set>
using namespace std;

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