#include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

using ll = long long;

template <class T>
using grid = vector<vector<T>>;

#define REP(i,n) for(ll i=0;i<(ll)(n);i++)
#define REPD(i,n) for(ll i=n-1;i>=0;i--)
#define FOR(i,a,b) for(ll i=a;i<=(ll)(b);i++)
#define FORD(i,a,b) for(ll i=a;i>=(ll)(b);i--)

#define input(...) __VA_ARGS__; in(__VA_ARGS__)

void print() {
  std::cout << std::endl;
}

template <class Head, class... Tail>
void print(Head&& head, Tail&&... tail) {
  std::cout << head << " ";
  print(std::forward<Tail>(tail)...);
}

void in() { }

template <class Head, class... Tail>
void in(Head&& head, Tail&&... tail) {
  cin >> head;
  in(std::forward<Tail>(tail)...);
}

int main() {
  ll input(A, B);

  if (A == 1 || B == 1) {
    print(0);
    return 0;
  }

  /* if (A % B == 0 || B % A == 0) { */
  /*   print(-1); */
  /*   return 0; */
  /* } */

  vector<ll> result;
  REP(x, 3000) {
    REP(y, 3000) {
      result.push_back(A * x + B * y);
    }
  }

  sort(result.begin(), result.end());
  ll p = 0;
  ll check = 0;
  int ans = 0;
  for(auto v: result) {
    int range = max((v - p) - 1, 0ll);
    if (range > 0) {
      if (v - check > 1000) {
        print(ans);
        return 0;
      }
      check = v;
    }
    ans += range;
    p = v;
  }
  print(-1);
}