#include <iostream>
using namespace std;
int gcd(int a,int b){
  if(a%b)return gcd(b,a%b);
  return b;
}
int main(){
  int a,b;cin>>a>>b;
  if(a>b)swap(a,b);
  if(gcd(a,b) != 1){
    cout << -1 << endl;
    return 0;
  }
  int ans = 0;
  for(int i = 0; a > i; i++){
    ans += (b*i)/a;
  }
  cout << ans << endl;
}