#include<bits/stdc++.h>
using namespace std;
using Int = long long;
template<typename T1,typename T2> inline void chmin(T1 &a,T2 b){if(a>b) a=b;}
template<typename T1,typename T2> inline void chmax(T1 &a,T2 b){if(a<b) a=b;}


template<typename T> void drop(const T &x){cout<<x<<endl;exit(0);}

//INSERT ABOVE HERE
signed main(){
  int a,b;
  cin>>a>>b;
  if(__gcd(a,b)!=1) drop(-1);

  const int MAX = 1e4+10;
  vector<int> ok(MAX,1);
  for(int i=0;i<MAX;i++)
    for(int j=0;j<MAX;j++)
      if(i*a+j*b<MAX) ok[i*a+j*b]=0;

  int ans=0;
  for(int i=0;i<MAX;i++)
    ans+=ok[i];
  cout<<ans<<endl;
  return 0;
}