#include <algorithm>
#include <bitset>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <vector>
#define MOD (1000000007l)
#define ll long long
#define rep(i, n) for (ll i = 0; i < (n); i++)

using namespace std;

ll calc_gcd(ll a, ll b) {
  while (true)
    if (a)
      b %= a, swap(a, b);
    else
      return b;
}

ll calc_lcm(ll a, ll b) { return a * b / calc_gcd(a, b); }

void solve() {
  ll A, B;
  cin >> A >> B;

  if (A == 1 or B == 1) {
    cout << 0 << endl;
    return;
  }

  ll gcd = calc_gcd(A, B);
  if (gcd != 1) {
    cout << -1 << endl;
    return;
  }

  ll lcm = calc_lcm(A, B);
  vector<bool> foo(lcm+1, true);
  foo[0] = false;

  rep (i, B + 1) rep (j, A + 1) {
    ll tmp = A * i + B * j;
    if (tmp <= lcm) foo[tmp] = false;
  }

  ll ans = 0;
  for (auto a: foo) if (a) ans++;
  cout << ans << endl;
}

int main(void) {
  cin.tie(0);
  ios::sync_with_stdio(false);
  cout.precision(12);
  cout << fixed;
  solve();
  return 0;
}