#include<bits/stdc++.h>
using namespace std;
#define ALL(x) begin(x),end(x)
#define rep(i,n) for(int i=0;i<(n);i++)
#define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl;
#define mod 998244353
using ll=long long;
const int INF=1000000000;
const ll LINF=1001002003004005006ll;
int dx[]={1,0,-1,0},dy[]={0,1,0,-1};
template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;}
template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;}

struct IOSetup{
    IOSetup(){
        cin.tie(0);
        ios::sync_with_stdio(0);
        cout<<fixed<<setprecision(12);
    }
} iosetup;
 
template<typename T>
ostream &operator<<(ostream &os,const vector<T>&v){
    for(int i=0;i<(int)v.size();i++) os<<v[i]<<(i+1==(int)v.size()?"":" ");
    return os;
}
template<typename T>
istream &operator>>(istream &is,vector<T>&v){
    for(T &x:v)is>>x;
    return is;
}

int b(int x,int y){
    return ((x>>y)&1);
}

int g(int a,int k){
    int ret=0;
    rep(i,k){
        for(int j=i+1;j<=k;j++){
            bool flag=true;
            for(int t=0;t<j-i;t++){
                if(b(a,i+t)!=b(a,j-1-t)) flag=false;
                if(flag) chmax(ret,j-i);
            }
        }
    }
    return ret;
}

void test(){
    const int n=9;
    int res=INF;
    for(int i=0;i<(1<<n);i++){
        auto gr=g(i,n);
        chmin(res,gr);
        cout<<bitset<n>(i)<<" : "<<g(i,n)<<endl;
    }
    cout<<"!, "<<res<<endl;
}

signed main(){
    // test();

    int n,k;cin>>n>>k;
    if(2*k<n){
        cout<<-1<<endl;
        return 0;
    }
    assert(false);
    return 0;
}