#include<bits/stdc++.h>
using namespace std;

long long a,b,aux;

inline long long cmmdc(long long a,long long b) { return !b ? a:cmmdc(b,a%b); }

int main()
{
  ios_base::sync_with_stdio(0); cin.tie(0);

  cin>>a>>b;

  if(a<2) return cout<<b-a+1<<'\n',0;

  if(cmmdc(a,b)==1) return cout<<b-a+1<<'\n',0;

  if(b%a) aux=b/a,aux*=a,++aux;
  else aux=b-1;

  if(cmmdc(a,aux)==1) return cout<<aux-a+1<<'\n',0;

  cout<<"-1\n";

 return 0;
}