#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <queue>
#include <iomanip>
#include <set>
#include <tuple>
#define mkp make_pair
#define mkt make_tuple
#define rep(i,n) for(int i = 0; i < (n); ++i)
using namespace std;
typedef long long ll;
const ll MOD=1e9+7;

int gcd(int a,int b){
  return b==0?a:gcd(b,a%b);
}

int main(){
  int A,B;
  cin>>A>>B;
  if(A>B) swap(A,B);

  int g=gcd(A,B);
  if(g!=1){
    cout<<-1<<endl;
    return 0;
  }

  int ans=0;
  for(int i=1;i<=A*B;i++){
    bool ok=false;
    for(int j=0;j<=B;j++){
      int rest=i-A*j;
      if(rest<0) continue;
      if(rest%B==0) ok=true;
    }
    if(ok==false) ans++;
  }
  cout<<ans<<endl;

  return 0;
}