#include <bits/stdc++.h>
using namespace std;
#define REP(i,a,n) for(int i=(a); i<(int)(n); i++)
#define rep(i,n) REP(i,0,n)
#define FOR(it,c) for(__typeof((c).begin()) it=(c).begin(); it!=(c).end(); ++it)
#define ALLOF(c) (c).begin(), (c).end()
typedef long long ll;
typedef unsigned long long ull;


int main(){
  int n, m;
  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;
}