#include <bits/stdc++.h>
using namespace std;

typedef unsigned long long ull;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef pair<double, double> pdd;
//const ull mod = 1e9 + 7;
const ll mod = 1e9 + 7;
#define REP(i,n) for(int i=0;i<(int)n;++i)

// debug
#define dump(x)  cerr << #x << " = " << (x) << endl;
#define debug(x) cerr << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;

template<class S, class T> ostream& operator << (ostream& os, const pair<S, T> v){
  os << "(" << v.first << ", " << v.second << ")"; return os;
}
template<class T> ostream& operator << (ostream& os, const vector<T> v){
  for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << " ";} os << v[i];} return os;
}
template<class T> ostream& operator << (ostream& os, const vector<vector<T>> v){
  for(int i = 0; i < (int)v.size(); i++){if(i > 0){os << endl;} os << v[i];} return os;
}
template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

// conversion number to bit
string num2bit(ll num, ll len){
  string bit = "";
  REP(i, len){
    bit += char('0'+(num>>i & 1));
  }
  return bit;
}

int main(){
    cin.tie(0);
    ios::sync_with_stdio(false);

    ll N, K;
    cin >> N >> K;

    if(N<K){
    	cout << -1 << endl;
    	return 0;
    }

    cout << K-2 << endl;
    return 0;
}