/* Aa^~ kokoro ga pyonpyon suru n jaa^~
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*/

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

using VI = vector<int>;
using VVI = vector<VI>;
using PII = pair<int, int>;
using LL = long long;
using VL = vector<LL>;
using VVL = vector<VL>;
using PLL = pair<LL, LL>;
using VS = vector<string>;

#define ALL(a)  begin((a)),end((a))
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define MT make_tuple
#define SZ(a) int((a).size())
#define SORT(c) sort(ALL((c)))
#define RSORT(c) sort(RALL((c)))
#define UNIQ(c) (c).erase(unique(ALL((c))), end((c)))

#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

#define FF first
#define SS second

template<class S, class T>
istream& operator>>(istream& is, pair<S,T>& p){
  return is >> p.FF >> p.SS;
}
template<class T>
istream& operator>>(istream& is, vector<T>& xs){
  for(auto& x: xs)
	is >> x;
  return is;
}
template<class S, class T>
ostream& operator<<(ostream& os, const pair<S,T>& p){
  return os << p.FF << " " << p.SS;
}
template<class T>
ostream& operator<<(ostream& os, const vector<T>& xs){
  for(unsigned int i=0;i<xs.size();++i)
	os << (i?" ":"") << xs[i];
  return os;
}
template<class T>
void maxi(T& x, T y){
  if(x < y) x = y;
}
template<class T>
void mini(T& x, T y){
  if(x > y) x = y;
}
void debug(istringstream&){}
template <char sep=',', class Head, class... Tail>
void debug(istringstream& iss, Head&& head, Tail&&... tail)
{
  string name;
  getline(iss, name, ',');
  cout << sep << name << "=" << head;
  debug(iss, forward<Tail>(tail)...);
}

#ifdef PYONPOI
#define DEBUG(...) \
  do{ \
    istringstream ss(#__VA_ARGS__); \
    debug<' '>(ss, __VA_ARGS__); \
    cout<<endl; \
  }while(0)
#else
#define DEBUG
#endif


const double EPS = 1e-10;
const double PI  = acos(-1.0);
const LL MOD = 1e9+7;

template<class T>
T ext_gcd(T a, T b, T& x, T& y){
  T d = a;
  if(b == 0){
    x = 1;
    y = 0;
  }
  else{
    d = ext_gcd<T>(b, a%b, y, x);
    y -= (a / b) * x;
  }
  return d;
}

template<class T>
bool ext_gcd_n(T a, T b, LL c, T& x, T& y, bool zero=true) {
  auto g = ext_gcd(a, b, x, y);
  if(c % g != 0) return false;
  LL d = c / g;
  x *= d;
  y *= d;
  auto dx = b / g;
  auto dy = a / g;

  auto t = T{0};
  if(x < 0)
    t = -x / dx + 1;
  else
    t = -x / dx;

  x += t * dx;
  if(!zero && x == 0){
    ++t;
    x += dx;
  }

  y -= t * dy;
  return (y < 0 || (y == 0 && zero));
}

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

  LL A, B;
  cin >> A >> B;
  if(B % A == 0){
    cout << -1 << endl;
  }
  else{
    LL ans = 1e18;
    for(LL a=1;a<A;++a){
      LL x, y;
      if(ext_gcd_n(A, B, a, x, y, false)){
        DEBUG(a, A, x, B, y);
        LL g = A * x + B * y;

        if(x < a - y){
          DEBUG(a, A, x, B, y);
          mini(ans, x * A);
        }
      }
    }
    cout << (ans < 1e18 ? ans : -1) << endl;
  }

  return 0;
}


Measured memory_bytes: 3208