#include <bits/stdc++.h>
#define REP(i, n) for (int i = 0; i < n; i++)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define ALL(v) (v).begin(), (v).end()
#define coutd(n) cout << fixed << setprecision(n)
#define ll long long int
#define vl vector<ll>
#define vi vector<int>
const int INF = 2147483647;
const ll MOD = 1000000007;

using namespace std;

int gcd(int n, int m) { return n ? gcd(m % n, n) : m; }

int main() {
  int a, b;
  cin >> a >> b;
  int g = gcd(a, b);
  if (g != 1) {
    cout << -1 << endl;
    return 0;
  }
  if (a == 1 || b == 1) {
    cout << 0 << endl;
    return 0;
  }

  vector<bool> ans(a * b, true);
  FOR(i, 1, a + 1) {
    for (int j = 0; b * i + a * j < a * b; j++) {
      ans[b * i + a * j] = false;
    }
  }
  FOR(j, 1, b + 1) {
    for (int i = 0; b * i + a * j < a * b; i++) {
      ans[b * i + a * j] = false;
    }
  }

  int tmp = 0;

  FOR(i, 1, a * b) {
    if (ans[i])
      tmp++;
  }
  cout << tmp << endl;

  return 0;
}