#include <bits/stdc++.h>
#include <atcoder/all>
#pragma GCC target ("avx")
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
using namespace std;
using namespace atcoder;
using ll=long long;
#define rep(i,a,b) for(it i=(it)(a);i<=(it)b;i++)
#define nrep(i,a,b) for(it i=(it)(a);i>=(it)b;i--)
#define all(v) v.begin(), v.end()
#define rall(v) v.rbegin(), v.rend()
#define moda 998244353LL
#define modb 1000000007LL
#define gyaku 166374059LL
#define dai 2500000000000000000LL
#define aoi 1e+18
#define tyu 2500000000
#define giri 1000000000
#define en 3.14159265358979
#define endl '\n'
#define sho -dai
#define eps 1e-14
#define yn(x) cout<<(x ? "Yes\n":"No\n");
#define YN(x) cout<<(x ? "YES\n":"NO\n");
#define cou(x) cout<<x<<endl;
#define ci(x) cin>>x;
#define fi(x) cout<<fixed<<setprecision(x);
using it=long long;
using un=unsigned long long;
using db=long double;
using st=string;
using ch=char;
using bo=bool;
using P=pair<it,it>;
using ip=pair<int,int>;
using mint=modint;
using mints=modint998244353;
using minto=modint1000000007;
using vi=vector<it>;
using ivi=vector<int>;
using vd=vector<db>;
using vs=vector<st>;
using vc=vector<ch>;
using vb=vector<bo>;
using vp=vector<P>;
using ivp=vector<ip>;
using vm=vector<mint>;
using vms=vector<mints>;
using vmo=vector<minto>;
using sp=set<P>;
using isp=set<ip>;
using ss=set<st>;
using sc=set<ch>;
using si=set<it>;
using isi=set<int>;
using svi=set<vi>;
using vvi=vector<vi>;
using ivvi=vector<ivi>;
using vvd=vector<vd>;
using vvs=vector<vs>;
using vvb=vector<vb>;
using vvc=vector<vc>;
using vvp=vector<vp>;
using ivvp=vector<ivp>;
using vvm=vector<vm>;
using vvms=vector<vms>;
using vvmo=vector<vmo>;
using vsi=vector<si>;
using ivsi=vector<isi>;
using vsp=vector<sp>;
using ivsp=vector<isp>;
using vvsi=vector<vsi>;
using ivvsi=vector<ivsi>;
using vvsp=vector<vsp>;
using ivvsp=vector<ivsp>;
using vvvi=vector<vvi>;
using ivvvi=vector<ivvi>;
using vvvd=vector<vvd>;
using vvvm=vector<vvm>;
using vvvvi=vector<vvvi>;
using ivvvvi=vector<ivvvi>;
const it dx[4]={0,1,0,-1};
const it dy[4]={1,0,-1,0};

template<typename T>
void scan(vector<T> &a){
  rep(i,0,a.size()-1)cin>>a[i];
}

template<typename T>
void scan(vector<T> &a,vector<T>&b){
  rep(i,0,a.size()-1)cin>>a[i]>>b[i];
}

template<typename T>
void scan(vector<pair<T,T>> &a){
  rep(i,0,a.size()-1)cin>>a[i].first>>a[i].second;
}

template<typename T>
void dec(vector<T> &a){
  rep(i,0,a.size()-1)a[i]--;
}

it gcda(it a,it b,it x){
  //xは使わない
  if(!a||!b){
    return max(a,b);
  }
  while(a%b&&b%a){
    if(a>b)a%=b;
    else b%=a;
  }
  return min(a,b);
}

it lcma(it a,it b,it x){
  //xは使わない
  return a/gcda(a,b,x)*b;
}

bo ooz(st &s,int n){
  for(int i=0;i<n;i+=3)
    if(s.substr(i,3)!="110")return false;
  return true;
}

int main(){
  ios::sync_with_stdio(false);
  cin.tie(nullptr);
  int n,k;cin>>n>>k;
  cout<<(k>n?-1:k-1)<<endl;
}