#include<bits/stdc++.h>

using namespace std;
using ll=long long;
#define rep2(i, a, n) for(int i = (a); i < (n); i++)
#define rep(i, n) rep2(i,0,n)

int main(){
  cin.tie(nullptr);ios_base::sync_with_stdio(false);
  int a,b;cin>>a>>b;
  if(__gcd(a,b)!=1){
    cout<<-1<<endl;
    return 0;
  }
  vector<bool> ok(a * b + 1, 0);
  rep(i,b+1){
    rep(j,a+1){
      int n=a*i+b*j;
      if(n<=a*b)ok[n]=1;
    }
  }
  int res=0;
  rep(i,a*b+1)if(!ok[i])res++;
  cout<<res<<endl;
}