#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define inf 10e17
#define rep(i,n) for(long long i=0; i<n; i++)
#define repr(i,n,m) for(long long i=m; i<n; i++)
#define mod 1000000007
#define sorti(x) sort(x.begin(), x.end())
#define sortd(x) sort(x.begin(), x.end(), std::greater<long long>())
#define debug(x) std::cerr << (x) << std::endl;
#define roll(x) for (auto&& itr : x) { cerr << (itr) << " "; }

template <class T> inline void chmax(T &ans, T t) { if (t > ans) ans = t;}
template <class T> inline void chmin(T &ans, T t) { if (t < ans) ans = t;}

// 最大公約数を求める(ユークリッドの互除法).
long long gcd (long long x, long long y) {
  if (y > x) swap(x,y);
  if (y == 0) return x;
  return gcd(x%y,y);
}

int main() {
  int a,b;
  cin >> a >> b;

  if (gcd(a,b) != 1) {
    cout << -1 << endl;
    return 0;
  }

  int const mmax = a * b;
  int ans = 0;
  map<int, bool> mp;
  for (int x = 0; x * a <= mmax; ++x) {
    for (int y = 0; x * a + y * b <= mmax; ++y) {
      mp[x * a + y * b] = true;
    }
  }

  for (int i = 0; i <= mmax; ++i) {
    if (!mp[i]) ans += 1;
  }

  cout << ans << endl;
}