#include <bits/stdc++.h>
using namespace std;
 
int main() {
  long long a,b;cin >> a >> b;
  if (min(a,b)==1) cout << 0 << endl;
  else if (__gcd(a,b)>1) cout << -1 << endl;
  else {
      long long count=a*b-1;
      for (int i=0;i<=a;i++) {
          for (int j=0;j<=b;j++) {
              if (i+j>0 && a*j+b*i<a*b) count--;
          }
      }
      cout << count << endl;
  }
}