#include <bits/stdc++.h>
#include <typeinfo>
#include <cxxabi.h>
#ifdef LOCAL
#include "dbgtoki.hpp"
#define DUMP(i) dump((string)TOSTRING(i), demangle(typeid(i).name()), __LINE__ , i) 
#else
#define DUMP(i)
#endif

using namespace std;
#define TOSTRING(x) #x
#define SZ(x) (int)(x).size()
#define REP(i,n) for(int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define ALL(s) (s).begin(), (s).end()
#define UNIQUE(v) v.erase( unique(v.begin(), v.end()) , v.end());
#define dis distance
#define so(V) sort(ALL(V))
#define rev(V) reverse(ALL(V))
#define pub push_back
#define sec second
#define fir first

typedef long long unsigned int llu;
typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vector <int> > vvi;
const int EPS = 1e-9;
const int MOD = 1e9+7;
const int INF = (1 << 30);

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);
	int n,m;
	cin >> n >> m;
	if(n <= m){
		cout << 1 << endl;
	}
	else if(n % 2 == 1){
		cout << -1 << endl;
	}
	else if(n/2 <= m){
		cout << 2 << endl;
	}else{
		cout << -1 << endl;
	}
	return 0;
}