#include <iostream>
using namespace std;
long long a, b, i;
long long cmmdc(long long A, long long B)
{
	long long c=A%B;
	while(c)
	{
		A=B;
		B=c;
		c=A%B;
	}
	return B;
}

int main()
{
	cin>>a>>b;
	while(b>a)
	{
		for(i=a; i<b; i++)
		{
			if(cmmdc(i, b)==1LL)
			{
				cout<<b-i+1;
				exit(0);
			}
			
		}
		b--;
	}
	cout<<-1;
}