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

long long a,b,i,j,rs=-1;

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;

  for(i=a;i<=a+1e4;++i)
   for(j=b;j>=b-1e4;--j)
   if(i<=b && j>=a && i<=j && cmmdc(i,j)==1) rs=max(rs,j-i+1);

  cout<<rs<<'\n';

 return 0;
}