#include <bits/stdc++.h>

using namespace std;

typedef long long ll;


inline ll gcd(ll a, ll b) {
  if (!b) return a;
  return gcd(b, a%b);
}

int main() {
  ll a,b;
  cin>>a>>b;

  ll d;
  for (d=b-a;d>=1;d--) {
    for (ll s=a; s+d<=b; s++) {
      if (gcd(s,s+d)==1) {
	cout<<d+1<<endl;
	return 0;
      }
    }
  }
  cout<<-1<<endl;
  
  return 0;
}