#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstdio>
#include <cstring>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <algorithm>

#define CK(N,A,B) (A<=N&&N<B)
#define REP(i,a,b) for(int i=a;i<b;i++)
#define RREP(i,a,b) for(int i=(b-1);a<=i;i--)
#define F first
#define S second
#define ll long long

const int INF = 1e9;
const long long  LLINF = 1e15;

using namespace std;
int N, M;
int main(){
  cin>>N>>M;
  if(N <= M) cout<<1<<endl;
  else if(N%2 == 0 && N/2 <= M) cout<<2<<endl;
  else cout<<-1<<endl;

  return 0;
}